FOXHOUND was about a thousand times cooler than Dead Cell.. Simple as that, really. There seemed to just be more effort put into the implementation of FOXHOUND and there was more history to them. Plus the stories revolving around FOXHOUND had that whole internal/external both conflicted feel.. Dead Cell didn't even make sense until halfway through Sons of Liberty. And they never even explained why the hell Vamp could do all that stuff. Yah, he drank blood, so he might have a taste for it, yah he's Bi.. big deal? Why does a bullet to the head NOT KILL THE BASTARD? Why can he run on water and jump 50 feet and all that crap?
Of course that's just my opinion.