Was just doing some rough sums with a little bit of rounding:
At level 16 with 22 Might and Sandals of the Forgotten Friar Fists =
10 – 15 Damage * 1.36 (Might) + .2 (Savage Attack) + .15 (Sneak Attack) = 1.71 modifier
+12 (+2) Damage = *1.36 = 19
= 17 – 25 + 19 Damage
= 36 – 44 Damage @ +18 Accuracy
Legendary Bittercut under the same circumstances =
11 – 16 * 1.36 (Might) + .55 (Legendary) + .2 (Savage Attack) + .15 (Sneak Attack) + .2 (Spirit of Decay) + .2 (Sabre)
11 – 16 * 2.66 damage modifier
= 29 – 42.5 + Lash@1.3 (9 – 13 extra damage) @ 15 Accuracy
Fists give you an average initial damage roll of 40 and Legendary Bittercut gives you an average initial damage roll of 36, though, of course, you're able to throw a an additional lash on top of bittercut because it's enchantable.
Just doing some sums based on these averages it seems that fists will, in general, give you more bang for your buck in terms of unmodified damage the more lashes you can stack on em'. However the fact that Bittercut's got that tasty dual Corrode/Slash damage can't be discounted.
Fists are still way faster than Bittercut, but that's not really that relevant under this build which is primarily designed to belt out Torment's Reach. Also don't forget the boot slots could be replaced with something else while using bittercut i.e. Shod-in-Faith.
So, if you're lazy like me and you can't be bothered micromanaging your monk to belt out Torment's reach all the time, or want to save on enchanting materials or give bittercut to someone else, fists will probably work a fair bit better for you imo (assuming you don't dump dex). If you don't mind spamming TR, which also means you can dump dex a bit, I'd say Bittercut likely has the edge.
Fortunately switching between the two isn't really that much of a problem as to use both you really only need Spirits of Decay and Weapon Focus: Ruffian. There's also those accuracies issues but I'm not sure exactly what the problem is.
Edited by Livegood118, 17 November 2016 - 05:20 PM.