The Book of Vile Darkness for 3e introduces statblocks for both demon princes and archdevils, and with a couple exceptions, they all have HDs in the mid-to-high 30s.
Then Fiendish Codex I comes along and says "nah, in 3.5 all the demon lords–princes included–have hit dice in the mid 20s. If you want to make them epic so that your epic-level players can enjoy a tough fight, though, here are some tips on how to do that."
Obviously with fewer HDs, the demon lords have lower hit points, attack bonuses, and saves. So, okay. It seems like 3.5 just doesn't want them to be quite as strong. Maybe 3.5 just squashes things down a little compared to 3e–it's been a while since Deities and Demigods, so it's possible that if the entities in that book were "updated to 3.5 standards", they'd be weaker, too.
But then, in Fiendish Codex II, it basically says "these are only aspects for archdevils, because the real things are way stronger. If you want to make actual stats for archdevils, here are some tips on how to improve these aspects."
And then the 3.5 archdevil aspects are at the same power level as 3.5 demon lords (if not slightly stronger), with basically the same guidelines to create "real archdevils" as the first book had to create "improved demon lords"!
So… What's the deal with that? In the DnD canon, are archdevils just way stronger than demon princes? Did this happen at some point in the settings' timelines between 3e and 3.5, or has it always been the case? Or did the bookwriters just change their minds midway through 2006 and decide to backpedal on giving nonepic archfiends, but never go back and change the now-weakened demon lords?
Best Answer
In D&D canon, the true personal power of both archdevils and demon princes isn’t nailed down. Same with deities. They’ve all been given contradictory statblocks many, many times (especially in separate adventures), and every time someone wants to write a new statblock (usually for a new adventure, where it is tailored to that adventure’s needs), the previous one gets hand-waved as just an aspect—but of course this time it’s the real thing, to make the adventure more impressive (until the next one).
Further, compare the Credits pages of Fiendish Codex I and Fiendish Codex II—none of the designers, editors, or development team have any overlap. There are managers who worked on both, but having separate teams makes a big difference. Further, Fiendish Codex I was June 2006, Fiendish Codex II was December 2006—there is a very good chance that the development of these books overlapped at least a little. (I do not have any personal knowledge of Wizards of the Coast’s development schedules for D&D 3.5e supplements, though.)
So you should put exactly zero weight on the statblocks for the various powers that be. None of those stats are relevant to the canon. You can’t even really contrast two related books like Fiendish Codex I and Fiendish Codex II as you have—D&D 3.5e supplements were written first-and-foremost to be stand-alone (or, stand-with-core, I guess). The dev teams for FC1 and FC2 simply came to different decisions about how to label things and there likely was very little effort made to try to get consistency.
As for what the actual canon says, it’s very hard to say. Both the archdukes of Hell and demon princes are very careful not to reveal all the power they have available. After all, the greatest personal threat that any of them face is their peers—they need to keep an ace or three or thirty up their sleeve just to survive. There is no canonical answer to who would win in a fight between most match-ups.
Things we do know
Archdukes of Hell
The nine archdukes each rule one of the Nine Hells of Baator. There is something of a “prestige” factor the deeper you go, meaning the title “Duke of the Seventh” is more prestigious than “Duke of the Third,” which might imply that the dukes of lower levels are more powerful than the dukes of the higher levels. This is definitely not a sure thing, however: archdukes are rarely, if ever, “promoted” to a deeper layer, and being an archduke offers a lot of opportunities for consolidating power, so a new archduke may well be weaker than the dukes of the levels above them. Furthermore, each layer has its own advantages that can override the prestige factor: Dispater would probably very much object to any attempt to remove him from the layer that’s named for him, even if a deeper layer might be “more prestigious,” because the Iron City is so valuable to Dispater’s plans.
Asmodeus has a plan to take all comers, archduke or otherwise. He has fended off numerous coups by Mephistopheles, and he has dealt with other past archdukes (Geryon, Levistus) rather summarily. It’s not clear, however, how much of this is his personal power, and how much of it is his connections, resources, and planning. Asmodeus is playing Xanatos speed chess very, very well. What his personal capabilities are is something he keeps very much secret.
Mephistopheles is generally presumed to be the second-most-powerful devil, after Asmodeus. He has made numerous attempts at taking Asmodeus’s position, and while he hasn’t been successful, he hasn’t been destroyed in the attempt, either. Asmodeus must have reasons for keeping him around—whether that’s because Mephistopheles is powerful enough to resist Asmodeus trying to oust him, or because Mephistopheles is powerful enough to be valuable to Asmodeus in his current position.
As an aside, Mephistopheles has a consort, Baalphegor. Very, very little is known about her, but she is known to have produced a vast portion of Hell’s innovations and improvements, from magic items and spells, to battlefield tactics, to organizational structure. There is a fan-theory that she is one of the ancient Baatorians, who had all-but-disappeared before Asmodeus and the rest ever got to the Nine Hells—this could make her extremely dangerous to Asmodeus, as she may have some kind of “right” to the Hells where Asmodeus is an interloper. As a Lawful plane, that could matter. She may be another reason why Asmodeus tolerates Mephistopheles.
It is not known why Baalphegor spends her time with Mephistopheles; though devils are—rarely—capable of love towards one another, it’s rare enough that in any devil relationship you expect to find some personal advantage in it for each member. It’s pretty obvious what Mephistopheles gets out of it, but it’s less clear what Baalphegor gets—because it’s utterly unclear what Baalphegor wants. (By the way, one of those rare exceptions was Asmodeus himself, who appears to have very strongly loved his consort, Bensozia, which explains his reaction to Levistus killing her.)
It’s pretty well-known that Bel is the weakest of the archdukes, as a recently-up-jumped pit fiend with limited resources, networking, and connections. In fact, Asmodeus appears to have promoted him to his position precisely because he wasn’t ready for it—it puts a huge target on his back and keeps him scrambling to hold it, preventing him from building up power, and it gives other ambitious devils something to focus on.
Demon Princes of the Abyss
There is no order to the Abyss, and the numbers on the layers don’t mean much of anything at all (except when, sometimes, they do). You definitely cannot judge a demon prince’s relative power solely by what layer they control, or even by how many layers they control—some of them control layers that nobody else really wants.
Demogorgon is probably the most powerful demon; he has the title Prince of Demons and has held that title for a long time. It’s a title that comes with a lot of power, and there are a lot of demons—including very powerful demon princes—who want to take it, so holding it for a long time speaks to a great deal of personal power.
Orcus, Graz’zt, and Lolth are probably on a similar level to one another, just below Demogorgon. They are constantly vying for control from one another, and none of them has “lost” that game yet, suggesting none of them is starkly stronger or weaker than the others.
Dagon is probably the most powerful demon that has ever been, though his power has likely waned somewhat since Chaos’s loss in the War of Law and Chaos. His support of Demogorgon’s claim to the Prince of Demons title is probably no small part of Demogorgon’s current standing.
Pale Night is mysterious in the extreme, and in some ways might fill a role similar to that of Baalphegor in Baator: she seems to be at work pushing the Abyss as a whole and all of demonkind towards greater power.
The Queen of Chaos, Obox-Ob, and Miska the Wolf-Spider are all effectively have-beens; Miska in particular is still trapped by the Rod of Law. See The Rod of Seven Parts adventure for more details on what happens if Miska is freed.
Pazuzu is weird; he claims all the skies above every layer of the Abyss. No one really challenges him on it but no one really recognizes it, either. He just kind of flies around being an evil bird, occasionally corrupting paladins.
Other demon lords are probably not quite as powerful of these, though it’s hard to tell for sure.
Note that Demogorgon, Graz’zt, Miska, Lolth, and Orcus are all Tanar’ri, while Dagon, the Queen of Chaos, Obox-Ob, and Pazuzu are Obyriths. Like the ancient Baatorians, the Obyriths were the original embodiments of Chaotic Evil, but they didn’t simply disappear. Instead, they were defeated in the War of Law and Chaos and the Tanar’ri rose up against them.
Things We Don’t Know
Much of anything between Hell and the Abyss. None of the archdukes or demon princes is really able to get at one another due to the planes between them, and so we can’t really deduce much about the fact that they haven’t killed each other yet. Demogorgon is probably the most personally powerful, but Asmodeus could probably figure out a way to kill him? At a guess?