We just went through this recently here, but the AR-10 has a greater potential, or at least higher average precision/accuracy than the M1A. It also tends to be considerably lighter, shorter, and less expensive. With less expensive and more readily available replacement parts, more easily serviced. I favor the M1A in my heart over the AR-10/LR-308, but I shoot and recommend AR's over M1A's.
There's a pretty good market for LR-308/DPMS Pattern rifles. The Armalite AR-10 pattern is locked up with much fewer licensees, but it's still not 100% single source. Lots of folks make barrels in various calibers, handguards, mags, receivers, etc for DPMS pattern. It's not nearly as standardized as the AR-15, but if you're looking to do a home brew or custom builder AR-10/LR-308, the DPMS pattern is the better choice.
I've owned both, but I'm down to only DPMS pattern rifles now, all 4 of them home built, two of them on Aero receiver sets. I've been tempted by an SR-25, but I don't love the handguard nor care to spend the money for piston operation - if and when Ruger releases the AR-25, I'm in for a receiver set. I have a 308win barrel for mine, but it's one's a 7-08, one a 260, and two 243wins. Been tempted to do a 6.5 creed or a 338fed, just haven't placed the barrel order yet.
My next battle rifle will be a SCAR 17, just to say I own it. Probably bonus time next year.