AR10 is a very accurate and ergonomic rifle, easy to field strip if you are familiar with the AR system. Accessories are easy to find, but spare parts are expensive. It can be a bit fussy with ammo, particularly milsurp, and the magazines are very expensive.
M1A is also accurate, though it lacks the potential accuracy of the AR10, and is comparatively light and handy for a 308. It has the best iron sights of any military semiauto IMHO. It is also very reliable. On the negative side, it is complicated to disassemble and requires a special lube, whereas the AR10 can make due with normal gun oil. Mags are expensive (though not as expensive as AR10 mags), spare parts are readily available but accessories run to the expensive side.
It's a hard decision, depends on what's more important to you. If accuracy is your main concern, the AR10 is more accurate and easier to scope. If reliability is your main concern, the M1A will feed most milsurp 7.62x51 where the AR10 may or may not.