Re: AR-10 or M&P.300 WHISPER ???????
Accurate when done correctly, but way too many problems to keep 'em running. That's onbe of the primary reasons that the M1As have fallen from favor in competitive circles, and the AR pattern rifles have replaced them.
Better off going with the AR platform right from the start, and not having to deal with the M1A headaches. Built right (good barrel, free floated and with a very good trigger), the AR should get the OP where he wants to go without much drama. Don't know what sort of LR shooting he wants to do, but setting up something along the lines of the AMU's M110s would be a good way to go. If you're not shooting it as a Service Rifle, that opens the options up a bit, and allows for some different caliber options, such as the 260.
several choices to be made here, but we need to know a bit more about this rifle's final application before we can give truly helpful advice.