TG has some of the best individual missions in the series, and its best missions are better than anything in T2, but T2 is a better overall game.
I didn't mind the zombies in the earlier levels (that one level in TG where you have to return the bodies to the graves and do a bunch of other stuff is one of my favorites, and that's all undead), but I think the last 2 or 3 missions of the game where it's all monsters and not many objectives other than "get to the end of the level" are awful. I got so bored with those levels that I just ran through as quickly as possible with a trail of monsters kiting behind me the whole way (which, honestly, is the best way to do them on expert. Stealthing or fighting through them would take ages and just be frustrating, I think).
System Shock 2 had that problem as well, though, in that it kind of degenerates into a really shitty FPS in it's last couple levels (and dark engine, while awesome at stealth, isn't a great combat engine). T2 is my favorite dark engine game by a mile just because combat is entirely optional (yeah, you could probably avoid combat in some of TG's undead/monster levels too if determined, but it makes things way more frustrating).