I just finished my UV run for DOOM II. I attempted a UV run through DOOM a year ago or so, but my game would freeze at certain points, typically after ending a level that would be followed by a text story. And now oddly enough I don't even have the full version. It claims to be free to download via the trial, but when I try to do so it comes up with an error code.
Anyways, DOOM seemed much harder, because of the lack of a super shotty. But aside from that one gleaming detail, I would Say DOOM II was harder. It's always gonna bug me that I never got that achievement for DOOM, but the Superior Firepower achievement eases the pain. :P