I shoot both, so I guess I would relate to your post more as which one would I keep? Very tough question!
They both have their advantages and drawbacks. Neither are my long range go-to rifles. If I can hit a 5 gallon bucket everytime at 400 yds, I'm good to go with them. Most all of my rifle shooting is from a bench off bags so I do favor high power magnification in my scopes, can't hit what you can't see.
My M1As are SA & the AR-10 is Armalite, all have lifetime warranties, equal on that point. The AR-10 is more ammo and mag sensitive than the M1A, but like JonM noted it's easier & cheaper, (cost of a quality mount) to scope.
If I could only have one, I'd have to keep the M1A due to reliability ONLY. In my experience, right from the box, the AR-10 has a slight edge in accuracy over the standard M1A. But I've never had a jam with my SAs, guess they win! This is all JMHO, I'm sure others will have their own ideas. Good Post!