I have a Rush 24, and I quite like it, but it's a bit on the heavy side, even unloaded. I also think it's a little on the large size for an 18-24 game. I'd lean toward the Rush 12, or something like a Tactical Tailor pack that's made of a lighter weight material.
Personally I'm eyeballing this:
http://www.skdtac.com/PIG-Runt-Urban-Pack-p/pig.702.htm
(This is assuming you're talking about a small pack to wear in game, to carry like bb's, mre's, extra magazines, or other small items you might need during the game.)