Here's what you're trading for the alertness feat if you don't take the abil score upgrade and instead take the feat:
- +1 to damage
- +1 to hit
- +1 to AC
- +1 to Dex saves
- +1 to Dex checks
- +1 to init
The question then, ultimately, becomes, is +5 init (net +4), no surprise and no advantage on attacks against from hidden opponents worth it.
The latter two features are very dependent on your campaign and DM. Surprise should be a fairly frequent occurrence on both sides of combat, but that still depends on how willing your DM is to allow it or stage his monsters to have them obtain it (and kind of how cautious your group tends to be). Monsters attacking from hidden is another one that really depends on your DM, you group and how well you guys are at nosing out monsters from the shadows.
So ignoring those two items since they are DM and campaign dependent (and thus can't be weighted objectively), the question then becomes, is +1 to Dex more or less beneficial over 4 levels than +5 to init.
For pretty much any character other than the Assassin Rogue, I'd probably argue that they should take the stat upgrade early, the feat at L8 and then the last stat upgrade. However +5 init has amazing synergy with the Assassin Rogue and you're going to get a ton of mileage out of it.
If you're already at 20 Dex (if you managed to start at 20, or 18 and are at L8), then there really isn't any stat upgrade that is worth taking over this feat (though there may be better feats). Rogues are skill monkeys, yes, but +5% in a single ability set of skills is not worth the +25% to initiative and other features o this feat. The only one that would be worth considering at all is Con for the extra HP, but it's really not that many and you're better of boosting your damage significantly.
Take the feat at L4 (or even better, play a human and take it at L1).
First up, the RAW. This is pretty simple: Divine Sense works on celestials, fiends, and undead. Player characters are all humanoids (see page 11 of the PHB), and the Tiefling traits do not say anything about making the character a fiend in any way.
Next, we have the lore. In 3.5e, tieflings were the descendants of actual fiends, whereas in 5e, they are simply a race that has had the essence of Asmodeus infused into their bloodline. So they're a bit less fiendish than they used to be. If we look at the fiend type in the Monster Manual, it says that
Fiends are creatures of wickedness native to the Lower Planes.
Tieflings definitely don't fall under this category lore-wise, because they are a humanoid race native to the Material Plane.
Trying to argue that abilities that work on fiends should work on tieflings is like trying to argue that abilities that work on dragons should work on dragonborn, but what you have to remember is that fiends are the physical embodiment of evil. The differences between tieflings and real fiends are insurmountably greater than the differences between dragonborn and dragons.
The taint in the tiefling bloodline is a devilish one, so if they were fiends, they would be devils. Taking a look at the description of devils in the Monster Manual, we see that
Devils personify tyranny,
and that
Devils live to conquer, enslave, and oppress.
The fact that tieflings are allowed to be any alignment contradicts this, and similarly, tiefling characters are allowed to do things other than conquer, enslave and oppress - they're definitely not devils.
Finally, if another argument is needed, Jeremy Crawford agrees that tieflings are humanoids, not fiends, and can't be sensed by a Paladin's Divine Sense.
Best Answer
Blindsight is is generally better than blindsense
The rogue's Blindsense ability states that:
Essentially, if there is a creature within 10 feet of you, regardless of how well it is hidden or if it is invisible, you can hear it and you know roughly where it is. Knowing a creature's location means you don't have to guess where it is, but if you can't actually see it that still has consequences; you can't target it with spells that specify a target you can see, you have disadvantage on attacks against it, that kind of thing.
In contrast, the Blind Fighting style states that:
For most purposes blindsight is better than just knowing where things are; it is the ability to functionally see within the specified range regardless of any other circumstances. This means you can target spells that require you to see a creature, attack without disadvantage, etc. For the most part that's a straight upgrade on the benefit of Blindsense.
However, there are a couple of things Blindsense can do which blindsight cannot. Blindsight is effectively blocked by total cover, but Blindsense isn't; as written, the rogue can hear enemies even behind walls and other such obstacles. Blindsight is also defeated by stealth, whereas Blindsense alerts you to creatures even if they have successfully hidden.
If you intend to be fighting in the dark a lot (or you fight a lot of invisible creatures), and you don't otherwise have any way to see in darkness (or see invisible things), you won't want to give up your blindsight once you get Blindsense. You need to be able to reliably negate the disadvantage you would get on attacks, otherwise you'll have a hard time triggering your Sneak Attack.