I like roper style because I feel they are more secure. I don't know how much of that is real vs. imagined. I also have some straight cinches that work just fine. But roper is my favorite.
Weather or not they bother the horse's armpit I think has a lot to do with the horse's conformation. If a horse is very round in the belly, the cinches seem to sit in the armpit more. But even with a round horse, I have never had mohair rub the skin. I've had them take off a little hair in the armpit of a Mustang I used to own, but they never made a sore on his skin. With my current horse, even though she is a bit of a chub at the moment, the cinch seems to stay out of her armpit, even the widest roper style.
I have one saddle that is probably close to fully rigged, maybe 7/8? I'm not sure, but it has the traditional rigging on the tree (vs. dropped rigging that is popular now) and even with that saddle, everything seems to slide into an appropriate place and doesn't bother her armpits.
So, I don't think you can go wrong either way. But if you notice all your cinches snug right into the armpit, you might do better with a straight style. Maybe.