[_paralyzed_]

the only thing you really gain with the xl is the wide axles. (they are widened outside the bearing carrier, arm length is the same, poor design IMO, I prefer flm wide arms)

the flux has the correct tranny, the xl doesn't

xl won't have motor mounts

the 8 shocks of the xl might handle the weight better than the 4 of the flux

IMO the flux would be an easier starting point, but then again, I'd build it ground up with flm combo bulks, hpi bulletproof diffs(which you'll want and come with neith the flux or xl) flm arms blah blah blah

but that's just me!