Depends on where your thumb is.
For example, this is the way I hold my controller. It allows me to access the a button, c buttons, and r all with the same finger
http://i.imgur.com/bnPEOnq.jpg
Since nerve impulses only travel around 250mph (at their fastest), it makes sense to utilize body parts that are closer to the brain, cutting down on the delay from brain to muscle. (However, I've been experimenting with holding the controller right in front of my face, decreasing the distance from my brain to hands, thus shortening the path for nerve impulses to travel, leading to decreased delay). In this case, my aerial attacks would come out faster with A than R, because I'm pressing it with the bottom of my thumb and not the tip.
Edit: picture won't imbed, so I linked it instead