Well, M1As are pretty lousy for mounting optics to. The gun was never intended to use them, and the "solutions" are all quite awkward. The Marines spent a bunch of money trying to scope up some surplus M14s to make "Designated Marksman Rifles", but they've abandoned the program because the rifles were such a maintanence nightmare.
Between the AR10 and PSS, the decsion is, which do you need more? Accuracy or rapid follow-up shots? The PSS will be more accurate, but not much more at all, and the AR10 will obviously give you up to 19 quick follow-up shots. Of course, if you want something bigger than .308 caliber, only the PSS will do.