I own both a FN-FAL and a DPMS AR-10.
The FAL is a fantastic gun. It’s super reliable. I’ve never had a jam and can’t remember having a FtF malfunction even.
The AR-10, not so much. One long shooting session on a dry lake the gun locked up tight out of battery. Because of the design of the AR family an out-of-battery bolt prevents disassembly of the rifle. (Now I know what the ‘forward bolt assist’ on the AR is form. Mine does not have one.)
I think the jam was more caused by the very fine grit blowing around the area and getting into the gun while shooting prone.
Advantages of the AR are the much better mounting of optics (after messing around with it I’ve kept my FN in steel sights), less perceived recoil - due to the in-line spring is my guess.
The FN (mine is the “Para” model) collapsing stock is great, and really makes the rifle portable. My AR has the full sized non-folding stock, which works well.
So, over all I can see that an improved AR could be a nice platform. Despite it’s weaknesses I find I use the AR-10 a lot more than the FAL, the ability to mount optics is key.
Were I to get a another semi-auto magazine fed .308 I might still go for an updated DPMS, as the newer ones have the forward assist and other nice upgrades. The accuracy of that gun is fantastic. As good as my best bolt action!
Have you tried DSAs FAL scope mount/dust cover? It made all the difference.
http://www.dsarms.com/c-697-scope-mount-accessories.aspx