I'm partial to the Sith Robes. I did two run-throughs on normal (forget the rank name) to unlock everything (yeah, guess I could have just replayed the last stage twice), and am currently working on a Sith Master run (however, MGS4 kinda stole me away a few levels in).
I was a little disappointed to find that the only increase of "difficulty" in Sith Master is that your enemies have more health and do more damage. They aren't any more sophisticated, its lazy difficulty (fairly common in most games, I suppose).