Yes, I definitely underestimated the critical chance because I forget to concatenate it. My 0.1 should have been 0.271. I made the same mistake with Archer, so both are going to do more damage.
We agreed 20 Strength, and I believe Defense style pays off more for GWM Champion than GWF. Because GWF will add 1-2 points of damage a turn, whereas Defense style will take AC to 19.
Average damage on a hit for a character with 20 STR, GWM w/ Greataxe:
+5 (STR 20 as agreed)
+10 (GWM)
+6.5 (GWM fighter takes Defense style)
=21.5
Is that reasonable? If so, then with the critical hit changes my spreadsheet suggests - against AC 17
.
If the Champion is level 10, he can have both GWF and Defense. There's no tradeoff. Though I'd still rather have GWF than Defense if I could only have one. Unless you're a half-orc, why are you taking a great axe instead of a greatsword? The greatsword does more damage. There's no reason to have a great axe.
Also, I'd suggest doing the math on Battlemaster and pretending the only maneuver he has is Precision. You'll probably want to assume the Battlemaster knows the AC of his target so you can come up with a set sequence to use the maneuver. E.g., if you miss by 4 or less, use Precision. Then you'd have to make an assumption on number of rounds of combat between short rests. You'll probably find that the Battlemaster does more damage than a Champion because of how valuable hitting is.
If you miss by 4 or less, you'll use Precision on 20% of your attacks and, therefore, can last 20 swings for your 4 superiority dice. If you limit Precision usage to only your Attack action swings (for easier math and because any hit on your Attack action increases the chance of dropping a foe to 0 and getting a bonus attack. Dropping a foe to 0 with your bonus attack does nothing extra) then you will last 6-7 rounds. If there are two combats between short rests you can have combats last 3-4 rounds. This is probably a reasonable assumption.
You'll hit on 70% of those 20% of attacks and increase your hit percentage 14%. Of course, you can do the same with SS. But if you do this with the Battlemaster I encourage you to come up with some estimation of bonus action attack chances.
A GWM, Precision-using, GWF-having, greatsword-wielding Battlemaster will do 39.0 dpr versus an AC 17 foe. How often will he drop a foe to zero? Will he intentionally choose to target weaker enemies just to trigger a bonus attack? (I would...). For simple math let's assume that he gets a bonus attack on 25% of rounds in addition to his roughly 15% chance for a critical on one of his three attacks. That's a bonus attack 40% of the time and an extra 3.9 pts of damage per round.
So that's 39.0 + 3.9 = 42.9 damage per round.