@OP:
Grab all 3 of them, they are all good games. BUT. They were buggy as hell in their release states, constant ctd's, weird ai behaviour, grafix glitches, etc etc.
THIS is a must-have to any of the games. It fixes a lot of things mentioned above, they are currently in the midddle or releasing a newer version. A must-have!
The grafix engine was great back in the day but is showing its age. The mod overhauls a lot of the visual components of the game, amongst other things.
Metro 2033 i've played, but not for years. Got boring pretty quickly imo.