I own an M1A Loaded, have shot lots of ARs, don't own one, if that says anything about my thoughts.
If you just get a base model AR-10, it will probably have similar accuracy to the M1A, maybe a shade better, but the AR will be cheaper to accurize than an M1A. Now with that being said, I just don't care for the feel of the AR. The M1A feels great in the hands, balances well, despite its weight, and is a dream to shoot. ARs just feel toyish to me. Soulless or something, I can't put my finger on it, but they don't feel the way a rifle should IMO. I know I'm in the minority here, but hey, what can I say? I like what I like.
I would try to shoot both if at all possible. At least go to a gun show and handle both platforms if you haven't already. You're about to spend a chunk of change, you want to make sure you like the rifle.
Jason