Both are good, accurate rifles.
Which to choose depends upon your needs and future desires. It's a hard choice, as the magazines cost about the same (good hi caps are ~$80). Edge to the M1A in cost for replacement parts.
If you're familiar with the maintenance of an AR15 and unfamiliar with a M1A, I'd say go with the AR10... same maintenance procedures, more or less, just .308-sized parts.
If you're wanting to get into competition, the AR10 has a slight edge in accuracy (M1A still has that op rod yanking things around) and recoil, but I do not believe you can shoot them in service rifle competitions. The cost of getting a competition-grade rifle is about the same.
To me, the main advantage of an AR10 is the same advantage I see with an AR15... get tired of the configuration or want to go from a weapon to a target rifle -- just pull the old upper and put on a new one. Harder to do that with an M1A.
If it makes you feel any better, I'm trying to decide myself. I already own an M1A, but due to sentimental reasons I've stored it away, and I'm looking for a shooter.