Sidearm hits for 110% 20% of the time. That's 22% extra damage as opposed to Maniac's 20%, which you only get with 5 stacks. Maniac is better if you are using it to build up for a Fury dump like Hammer though.
Before you judge me as a graphics whore for my opinion on Xenoblade, keep in mind my favorite game is Dwarf Fortress. Both are great games with bad graphics.