Both are fine, but very different, games. Titan Quest is probably the best of the Diablo-clones, better even than either Diablo in my opinion. AvP is an utterly unique FPS. Creepy, atmospheric, punishingly difficult and faster than Quake 3. Finishing the fifth marine level on hard is to this day my proudest video game achievement, and the skirmish mode absorbed many hours of my youth.
You really couldn't go wrong with either of them, but my vote is with AvP.