The M1A is ten times the rifle the AR10 is, BUT all said above is true. An M1A is going to cost your two or three times as much as an AR10 will and once you have the rifle, doing anything to it will be a PITA. When you buy an M1A, what you can do to it is very limited. Even mounting a scope is a bit of a challenge. The M1A is the kind of rifle that you buy because you like it and will shoot it the way it is. It's a battle rifle with limited use for anything else. Well, that last one is arguable, but it's an arguable point.
With the AR10, just think AR15, only bigger. Barrels, trigger, magazine, accuracy, optics...it's a playground. If I had to choose one, it would be the AR10.