My first instinct is to say the FAL, b ut I guess it kind of depends on what you are gonna be using it for, what kind of configuration you want, and what type of ammo you intend to shoot ($3-$4 a box surplus, or $15 a box or more Super Match Ulta Expensive Primo Loads).
If you're going with Plain Jane, no bull barrels or NM this, that, and the other thing, go FAL.
If you're going to go the whole 9 yards with every Match this and Custom that, then go with the AR 10.
Actually, since you already said you're gonna get both, go with the tricked out AR10 and plain FAL.
Get the FAL first. That way if for some reason you ever need the rifle for serious use, you've got one hell of a battle rifle, not some tricked out paper puncher that you cant just stuff any .308/7.62x51mm ammo into and expect it to work reliably.
Plus, if you get the FAL first, you might save yourself a whole whopping heap of money by NOT getting the AR-10. Then again, you probably WILL spend that whole whopping heap of money on mags, ammo, and of course, ANOTHER FAL!