Shifter
Two extremely polarized schools of thought for shifter selection. The Shimano rapidfire use the thumb to shift and allows braking while shifting. The Gripshift clan uses a twisting action and can change more gears in one movement than the rapidfire. The Gripshift might be a bit more crash resistant than the rapidfire.
Most of the rental bikes I've ridden come as Gripshift, so I'm familar with this type of shifter. It is a pretty intuitive system, you simply set the dial to point to the desired gear. Reviews suggested the SRAM Plasma to be a winner. I picked the 2000 model equivalent, the SRAM Rocket shifter.
Note: The Gripshift shifter must be compatible with the rear derailleur. The SRAM ESP shifters must use the SRAM derailleurs. Double check that your shifter/derailleur is compatible.