In true AR15.com fashion, if you can't decide, buy both.
If you're shooting the rifle with iron sights only, buy the M1A. Match rear sights give finer adjustments than the GI issued ones (1/2 MOA elevation & windage, instead of 1 MOA elevation/1 MOA windage). Additionally, the M1A is typically better balanced than an AR-10T (the latter is muzzle heavy unless the buttstock trap is filled with lead). 1/4-1/4 MOA iron sights can be mounted on an AR-10T, but the rifle itself needs to be tweaked to get weight and balance right - again, most factory AR-10Ts aren't properly balanced.
If you plan on scoping the rifle, buy the AR-10. On an AR-10T (or any similar rifle), securely mounting a scope is far easier - you don't need a seperate mount to which rings are attached. Nor should you need a cheekpiece with a scoped AR-10T.
Experience says M1As are more maintenance intensive than an AR-10. An M1A gas system needs to be disassembled and cleaned about every 500 rounds to remove carbon inside the gas piston. Fail to do so, and eventually, you run into stoppages.
Experience also says M1As/M14s break/lose parts more frequently, usually at inopportune times. Firing pins and extractors are common failure items, with safeties and triggers being the next most common. With match conditioned flash suppressors (taper reamed from the front), check the suppressor slots for gas cutting/cracks every few thousand rounds after about 5000 rounds.
First time I shot a rack grade M14 (US Navy), I tried handing the damned thing back after the first shot. Unloaded, a rack grade rifle weighs less than 8 lbs., and with M80 ball ammo (147 grain FMJBT @ 2750-2800 FPS), kicks hard. A match conditioned M14 weighs closer to 12-13 lbs, and is more pleasant to shoot, even if the bullets are heavier (168-175 grain Sierras @ 2600-2650 FPS). In contrast, the more in-line design of an AR-10 changes the recoil sensation - less felt recoil and muzzle climb.
You might note the Army's new M110 sniper rifle is an SR-25 derivative, and not M14/M1A based. User training, durability, and up to 60% parts commonality with the M16/M4 makes a big difference.