I never liked the "sloping" button layout personally. Don't know what it's like in practice, admittedly, but it seems like it forces your right arm to 'jut' out into the next player. This is more evident with three or more columns of buttons. (I know Sega did this with their Genesis/MD arcade controller and it seemed okay, but that's only a one player controller.)
I think straight or a slight curve/offset where you keep your arm to yourself may be a bit better.
Nonetheless, try it out on a scrap/cardboard mock-up and see if you like it.