24 gears is better.

read up on the "Drivetrain Devo" mini-series in the fall 2005 issues of Mountain Bike Action.

stuff like a skinner chain and smaller tolerances between cogs make just a little bit of dirt skip up the shifting. in the long run in terms of drivetrain reliability, 8 cogs in the rear (24 if you have three chainrings) is better.

as for which having a higher gear ratios, they vary with the cogs and chainrings you get with your bike. don't worry about that... your rockhopper can be just as fast as any 27 speed bike. I don't think there's enough difference anyway - what that matters is the rider.