Hmmm...I have a Belgian FAL and an AR10 carbine, so really I am partial to both guns.
The AR10 is more ergonomic, more accurate, and easier to scope.
The FAL is more reliable, simpler and the mags are cheaper.
If I had to choose just one to keep and I wasn't considering collectability (which would be a factor for me since mine is a Belgian preban but would not in your case since you're talking about buying a DSA postban) I guess I would choose the AR10, since I already have quite a few hicap mags for it.
The AR10 is more ergonomic, more accurate, and easier to scope.
The FAL is more reliable, simpler and the mags are cheaper.
If I had to choose just one to keep and I wasn't considering collectability (which would be a factor for me since mine is a Belgian preban but would not in your case since you're talking about buying a DSA postban) I guess I would choose the AR10, since I already have quite a few hicap mags for it.