Neither the Massoud or the SCAR-H are on the market though. Until I see one on the wall of the shop, it's nothing but empty promises and a nice display at SHOT Show. (I'm still waiting for Kel-Tec's bullpup, too, which was first unveiled at SHOT 2007. It's been nearly two years.)
Depending on the M14 you drop into the chassis, it'll potentially be more reliable. The M14 is a battle rifle. The SR-25 is a match rifle. That accuracy comes at a price.
(Now, if you go plunk a Springfield Supermatch prima donna rifle in there, this probably doesn't apply.
I don't know about cheaper...at least not with the SCAR. I've heard a price point thrown around that's close to three grand.
At SHOT 2008 someone had on display a Troy modified M14, complete with M203. That was a heafty beast!
As for the AR-10...personally, from the things I've seen and heard working in FBMG, I wouldn't buy an Armalite product at all. The DPMS LR-308 line is great, though, and very accurate. Different kind of rifle than the M14.
If I was going to accurize an M14 I think I'd drop the coin and get a J. Allen Enterprises stock. Otherwise, for a regular battle rifle, the standard stock is fine. Well, the Crane EBR SOPMOD stock is cool (and very well thought out), but it's pretty heavy.
My battle rifle preference remains firmly in the FAL camp, though I admit to a secret crush on the M14.