] Tough decision though.. I'd choose the M1A though for nostalgic reasons.
"Laws that forbid the carrying of arms ... disarm only those who are neither inclined nor determined to commit crimes. ... Such laws make things worse for the assaulted and better for the assailants; they serve rather to encourage than to prevent homicides, for an unarmed man may be attacked with greater confidence than an armed man." -- Thomas Jefferson
The flat-topped AR10 is a better platform on which to mount optics. There has not yet been a fully satisfactory optic mount for the M1A (the Brookfield Precision mount is tough and reliable, but puts the scope awfully high, making a good cheek weld difficult).
The "scout" configuration M1A's show promise, but the M1A is pretty much an iron-sight affair. That said, it has excellent iron sights and is a superb rifle in that role.
I had a M1A scout. Probably best for your multi purpose. I was satisfied with mine but I traded it for a N.M. M1A. Mags for AR10 (nice piece too) are modified m14 and rather pricey. However AR10's come in a basic model and retail for $1000, so you may find one for $900. Uh......given your choices, and what your demands will be, it's almost a coin toss.
Butt, I would go with M1A full size. Long range capabilities are outstanding and you will never lose money on investment.
I've shot, cleaned and handled both rifles, and I'd prefer the AR-10 for your purposes. Unless you're planning to shoot/plink targets 400-500 yards away, an AR-10 will be the best platform for a scope and bipod. There are plenty accessories to modify the gun with from Armalite.
If you really want a M1A, look into the new M25 Carlos Hathcock edition. It has a built in pictinny rail on the receiver, and the receiver itself is lugged so it can be braced rigidly in the stock. Gets 0.75 MOA out to 300 yards according to some reviews I've read. Problem is, it's $3000. However, it's the ultimate rugged semi-auto sniper rifle. The Scout model isn't bad either, but you'd have to use a "scout" style 2-3x powered scope on the front rail. It works great out to 300 yards.