on quick glance, to me Doom II looks pretty much the same - i never played Ultimate Doom, btw. the corridors look brighter and cleaner, so they may have "brightened" it up a bit - and, therefore the enemies are clearer - from a distance
. as soon as you get up on them though, they turn into big pixelated blobs - and there aren't a heckuva lot of pixels either!
however, that said, i know i'll be playing it and enjoying it a lot more than the N64 version, which i quit in frustration due to the controls. the XB version supports dual analog sticks, and the character moves very fluidly and easily, which makes a gigantic difference. i didn't go very far, so the jury will still be out.
i did take a quick look at Ultimate Doom - it's the same as, or worse than, Doom II, as far as graphics go, and the music is very, very old school. both can be played co-op, and deathmatch up to 4 players though. the explosions, or gas bombs (or whatever the giant green pixel cloud was) are pretty funny to watch.
good luck with your (apparent) decision, i'm glad i got the extras - i already had fun just taking a quick look!