Sure Hughes has ended some fights with submission, but that is Serra's specialty. Not saying Hughes is a slouch in the department I just think Serra is better. Hughes just has so much more experience than Serra and of course has pulled of more submissions than him, doesn't mean he is better. So if you put these to in a pure bjj match, you would pick Hughes over Serra
Serra's specialty is defensive BJJ in MMA, the only legit submission win he has is a triangle-choke over Kelly Dullanty---hardly a ground fighter.
In a pure grappling match I'd possibly go with Serra, but Hughes has a very under-rated and under-appreciated submission game. You have to remember as well that this isn't a PURE BJJ, this is MMA BJJ, and it boils down to who can adapt their ground game the best. Just looking at records alone, Matt Hughes' adaptation of grappling is superior on the offensive level.