FAL or M1A, and here’s why…
Parts.
The FAL was used all over, so you should be able to get replacement parts for years to come. The M1A was used by the US Military (as the M-14) so you can find plenty of parts for it too.
The Armlite version of the AR-10 is probably an excellent rifle. If parts weren’t an issue it might even be the best choice. But there’s no guarantee that it will even be in production in 10 years and there’s no reserve of military surplus parts and accessories to fall back on.
For the FAL and STG58 (which is just a FAL in Austrian clothing.)
http://www.dsarms.com/
And for the M1A
http://www.springfield-armory.com/prod-rifles-m1a.shtml
(One thing to remember is that you not only have to buy the rifle you also have to buy magazines and accessories. Check the prices on the FAL magazines from DSA. That’s one reason the FAL is a good value.)