AR10, in DPMS pattern:
Cost, you can get almost two AR10's for the cost of one M1A.
Mags, Magpul makes awesome AR10 mags, plentiful and cheap.
Optics, yes, you can mount a scope to an M1A, but it is much, much easier on an AR10.
Sights, most AR desinged back up sights will work on a AR10
Furniture, The options for setting up stocks, grips, and some forends are wide and varied.
Cons, and something to think about...
With a few exceptions, AR10's are not as accurate as a M1A, at least in the basic form. (Mine is just under 1 inch-5 shot group with 168gr match)
They are heavy, as noted above, especially pitson ran ones.
They are mostly all proprietary. While there is a lot of accesory interchangeability, there is no "standard" and each company ,makes one a smidge, or even a bunch different. Best results are found by choosing one brand, and sticking to that same brand when ordering things like bolts, carriers, trigger groups, and such.
That said, here is the one I put together, even with scope and mount, I am still under $1k. Palmetto PA10.