If you are truely interested in Grip shifters, then the price point becomes an issue. Beware of all SRAM Grip shifter units under the level of X.7 (stick to x.7, x.9 or x.0). The quality of SRAM components is outstanding at this level but not so good at lower levels.
Incidently, the Shimano rear trigger shifter will shift up to 3 gears per push of the trigger. They will only come back at one per though.