I've had both for some time. I'm one of the few ArmaLite owners who's happy with my AR-10. I bought an ArmaLite M15 and had trouble with it. Had nothing but trouble with the "new" AR-10 mags (took back every single one I bought for malfunctions).
Soooo: my recommendation is the M1A, either the full-size or Scout version. Avoid both the SOCOM I and II.