El Mariachi said:
What niche does this fill?
The "people who want to see Dune reprinted and don't mind if it gets a re-theme and a few rules changes along the way, especially considering the Herbert estate apparently won't sell the rights to have it done as Dune" niche. =P
Seriously, though, I would classify the game as a political war game. There's fighting involved, but most of the action is politically motivated by the players and how they interact.
El Mariachi said:
I've heard the game works best with 6 players; but if I have 6 players available for a game why would I want to choose Rex over Twilight Imperium? Is it a quicker game by any chance?
Is it quicker? Very. Twilight with 6 takes a good 6 hours, in my experience. Even when everyone knows what they're doing. Rex takes about 2 hours, tops. Maybe 3 if you have to constantly explain rules to newbies.
That aside, however, the two games are entirely different, except for their setting. Twilight Imperium is an epic, fleet-scale game of galactic domination whose progression generally depends on what objective cards come up. Rex is a unit-scale game territory control, with a healthy dose of inter-player economy to keep things interesting.
In TI3, betrayal is possible, but not usually encountered (again, in my experience) unless it's necessary for one player to pull ahead and win. In Rex, betrayal is par for the course. Not only are there rules that allow for your opponent's leaders to turn traitor, but there are (optional) rules to assist players in betraying an alliance at the end of the game, so you can play for the win together and then pull the rug out from under your friends' feet after the deal is done! The inability to trust anyone (even your own allies) is very much a part of the Rex experience.
BTW: The reason Rex plays best with 6 is because that way there are more actual players to interact with throughout the game. A large portion of the game is placed in the players' hands to let them do with as they see fit. This isn't a game where you just follow the rules and try to come out on top; this is a game where you MAKE the rules, and do so to your own advantage. Every faction (of the 6 in the game) which isn't present for a particular play is that much more control taken out of the players' hands and left to the rulebook to decide.