The Bushmaster BAR-10 was a Rock River design, and it is different than other AR-10s in that it was designed to use FAL mags. This resulted in one lug being removed from the bolt face to work with the FAL mag, which weakened the bolt, making it easier to break the bolt. Rock River now has the design back and is working on it (has been for nearly 2 years) and they say that they will not release it until the problems are worked out. I suspect that it won't ever be released.
If you are looking for a target/hunting/plinking rifle, either will work. Though I think that the AR platform will make it easier to use optics, and will be more accurate "out of the box".
If you are looking for a Battle Rifle, the M1a Standard is what you want. It will cycle a wider range of ammo, is tougher and has less critical parts to concern yourself with. I would not bother with the Loaded or Match of the M1a, as they will not cycle surplus ammo as reliablly as the Standard. The Standard will give you "out of the box" accuracy to reach out to 600 yards, so it should fit your needs.