Emotion aside, here are the facts: An AR-10 is easier to maintain, generally more accurate out of the box, far easier and far cheaper to accurize, and will handle a much wider pressure range of loads. An AR-10 is also far easier to scope. All of that said, the M1A/M14 just feels better in the hands and shoulders better for me. Despite their similar weights, M1As/M14s feel better balanced to me than AR-10s. So, my brain likes AR-10s, but my heart likes M14s. You won't go wrong with either, so get the one that you think you'll like best.
Ar-10 is also easier to silence. Speed reloads are easier w/ the ar-10. All of those reasons are why I sold my NM M1A and built a .308 AR. I had visions of using the M1A for 3-gun heavy metal but all those reasons are why I made the change. I expect to run the ar-10 in some 2-gun or 3-gun matches this summer. That said, I already miss my M1A even though I seldom shot it, and I see another in my safe... someday...
-rvb