In the interest of disclosure I should mention I own both an M1A and FAL and recommend them highly, and don't own an AR-10, but you may be better with the AR.
The AR-10 is going to be easy to accurize and stay accurate due to the free floating barrel and lack of bedding. Now with those Troy or other metal stocks bedding may not be an issue anymore, but the standard stocks on an M1A require rebedding on a regular basis to maintain accuracy. Scope mounting is also easier on the AR-10, and you can get a very nice trigger with a lower pull weight easier as well.
The primary advantages of the M1A are the reliability and ruggedness of the weapon. None of the .308 ARs have that sort of reputation. But if you're not going into battle, it probably won't matter. I probably wouldn't go with the Troy or other railed monstrosities on an M1A. There is a company out of California that makes very nice fiberglass stocks with an aluminum internal chassis to eliminate bedding that I would probably use instead. Expensive, but nothing for an M1A is cheap.
