Asking your gunsmith M1A or AR10 should get you a definitive reply. He should recommend the one that gives him more business in upgrades and work - the M1A. It takes a gunsmith to accurize, as so many of the operations are machinist grade work.
The AR10, oh no, not that POS, because YOU CAN WORK ON IT AT HOME. It's easy work, and most of it on the internet. That's the beauty of the Stoner design - he got the gunsmithing out of it so that a 23 year old armorer could pretty much do it all just replacing parts. Ironically, the tradition bound Army won't let him.
What does it take to upgrade each? Barrel - AR10, square the upper, insert the new barrel, tighten the barrel nut, and install the gas tube. A special vice block is optional, you can use jaw inserts for much less. M1A, a special block to hold the receiver is much more necessary, headspace gauges required, and setting the headspace to within +/- .0005 necessary. It's much more complicated.
Free floating the AR10 means installing a proprietary barrel nut, attaching the tube, and if it's equpped with a low profile gas block, done. M1A, the stock has to be inletted, glass epoxy used to fit the action, and that pesky barrel band removed or relieved - as long as a sling is attached and it can move the barrel, all is for nought. Slings on free float AR10's attach to the tube.
There's dozens of other details, the optics platform of the A3 type upper offers far superior scope mounting vs. the side mount cantilevered versions forced by the top eject exposed bolt of the M1A. And once done, the fact that the Stoner design has taken the top dozen places over the M1A for the last decade doesn't help. The AR is inherently cheaper, easier to work on, more accurate, and easier to use, too.
The M1A isn't the better choice.
