Alien Trilogy is gameplay-wise very derivative of Doom. However, it very much has an Aliens feel to it, which means that unlike Doom, its actually scary. It's got great looking graphics for the time it came out, fantastic music that I still have on my iPod, and wonderful level design. The biggest drawback against Alien Trilogy is simply that its not as good as Resurrection.
Resurrection, however, is simply superb. If you cheat. I've never known anyone to be able to get over the blistering difficulty of Res and make it to the end without cheating, mostly due to the frustratingly bad controls. However, if you take some of the frustrating edge off of the difficulty, you'll find a brilliant experience. The alien AI is simply first rate, the scripted events are uniformly chilling, and the entire game has a wonderfully horrifying sense of claustrophobia that makes it just about the ideal Aliens experience imo.
