With Singularity, the old, Soviet aesthetic was really interesting! It makes the player feel like an explorer who just stumbled upon time travel. The graphics were a lot more polished as well. Be sure to see the multiple endings of this game!
TimeShift was more generic and the story didn't make much sense, but it gave the player more direct time control. Plus, the game had a more tactical feel to it.
Besides time travel, the one thing in common with both games is that they were way too short. I remember after I defeated the final boss in Singularity I wanted to continue playing the game--but that was it.
If you're looking for another FPS game with graphics like TimeShift, I would suggest
Chaser. This was another game that fell under the radar, but I think it deserved a second chance. The levels in Chaser are massive and it will take a while to finish that game!