GWF let's you reroll the die if it's a 1 and 2, so in the instance of a Greatsword do you reroll only one of those die if both land on a 1 or a 2, or both? I imagine if you could reroll both it would say dice but maybe that's just me misreading it.
Let's work out some of the math here; the details of your build and your DM are going to influence how these numbers work out. Some assumptions I am making based on what you have stated.
- You said DM is letting you use flanking rules so I am assuming you are getting advantage on each attack so I am going to state that you are getting advantage on your rolls 75% of the time due to other skills and tactics you could be doing.
- I am assuming that your attack stat at this point is a 20 in any of the builds. I am ignoring barbarians right now who can get more then a 20 in a stat also I am ignoring belts or other items that can raise your stat above a 20.
- Finally we are going to ignore the Sun Blade because getting a hold of a specific magic item should not be a requirement for a build.
- This also assumes that your DM lets you roll your X number of attacks and decide to apply the sneak attack to the critical attack. The once per term requirement is very vague on this and should be clarified.
My first thought is that the thief build is going to be really top heavy; you are trying to get as many dice rolls as possible because you want that natural 20 critical to double the number of dice you are rolling for damage. I did a little look at monsters vs stats and I am trying to figure out a good threshold for when a die roll will hit and I feel like 13 on the die should hit based on average AC of monsters at that level. So 13 for die roll, 2 for weapon bonus, 6 for proficiency and 5 for stat. Now one could argue these numbers one way or another but I am going to be using them as the base because I need something to compare apples to apples.
So 13 + 2 + 6 + 5 = 26 for target AC to hit or 35% chance to hit and a 5% chance to crit. I used http://andrewgelman.com/2014/07/12/dnd-5e-advantage-disadvantage-probability/ as a reference for determining advantage so that translates to a 63.9% chance to hit and a 9.8% to critical. That makes it so 54.1% of the time you are going to do normal damage and 9.8% time you are going to do double damage. I basically am saying 2/3 of the time you are going to hit with your attack, really anything that gives you 2 attacks is going to let you get a sneak attack in during that round so long as you are within 5 feet of your target. This is also a pretty high number, some could argue that 10 or above would be a more realistic number then you are talking almost an 80% chance to hit with advantage.
Since you decided to dip into fighter instead of go 20 rogue you don't get Stroke of Luck but that would only be a factor depending on how much resting vs combat you would do. To ease math on calculating average damage I am assuming your weapon damage for rogue is a D6. If you are getting a D8 or better this just slightly makes your numbers better it just helps me to do my math faster.
Critical Hit - 18d6 + 2d6 + 5 + 2 = 77 Avg
Normal Hit - 9d6 + 1d6 + 5 + 2 = 42 Avg
GWF Critical Hit - 18d6 + 2d6 + 5 + 2 = 85 Avg
GWF Normal Hit - 9d6 + 1d6 + 5 + 2 = 46 Avg
The math was kind of surprising because of the limited reroll only on 1 or 2 and you could get a 1 after you rerolled a 2 but on average on 10d6 rolling a 1 or 2 and getting 4 extra damage on average.
After looking at the polearm master the numbers are basically the same for attack and damage except that you get another attack that you will get your critical damage against. Even though the base dice is a d4 that extra attack is what gets you the key advantage — do you want that extra damage from the reroll or the chance to attack again and be able to sneak attack? Frankly I'd rather take the 63.9% to do my sneak attack dice again then the extra 4 damage on average.
This basically follows the same rules as Polearm master: the feat is letting you get an extra attack with the a crossbow and I still think it is better then Polearm master because being able to attack at range is crucial; however, that depends on your flavor and what you like to do. However where this starts to break down is flanking and how your GM rules on flanking with a crossbow. If your GM rules you can't with a range weapon then you are stuck with polearm.
Bottom line your GM house rule about flanking is getting you the most bang for your buck here because you want to maximize that rule. The winner between all 3 is really hard to decide, and also depends on how your GM house rules critical hits and determining which one was a sneak attack. If you get to pick the extra attack is going to be always worth it. If you can't pick which attack to apply sneak attack damage to then they are really almost a wash because your extra critical chance vs the on average higher damage from your rolling is going to end up being almost a wash in the long term at high level.
If you use Great Weapon Fighting with a feature like Divine Smite or a spell like hex, do you get to reroll any 1 or 2 you roll for the extra damage?
The Great Weapon Fighting feature—which is shared by fighters and paladins—is meant to benefit only the damage roll of the weapon used with the feature. For example, if you use a greatsword with the feature, you can reroll any 1 or 2 you roll on the weapon’s 2d6. If you’re a paladin and use Divine Smite with the greatsword, Great Weapon Fighting doesn’t let you reroll a 1 or 2 that you roll for the damage of Divine Smite.
Also: As pointed out by Doval, Jeremy Crawford allows all weapon dices to be rerolled. This includes, for instance, a Frost Brand sword's additional cold damage, since that damage is part of the weapon's damage itself (not from an additional feature).