I always had GripShift on my mtb and loved it. Very intuitive, I thought.
When I got a new mtb, I had the shop swap out the rapid fire for GS.
Then I got a flat bar road bike with rapid fires, which I ride tons more than my mtb (as my commuter). Now, on those rare times when I find I got back to riding the mtb, I have trouble "remembering" how to shift!
It's very awkward to go back and forth b/t the two.
So...I'd say pick one and stick with it. If you have and like rapid fires, stay with it. If you have another bike that you ride that uses one or the other, I'd say get both bikes with the same!
Or, maybe those issues are just me!