Originally Posted by Oarhead
It sounds like he is going to be doing mostly PacNW coast style riding i.e. trails with some built up stuff, drops, steeps and skinnies. In my experience trying to do wheelie drops, bunny hops, manuals and throwing your body and bike around produces shifting with Gripshifters.
If I lived in california and id a lot of fast single/double track that was not built up and did not require a lot of body/grip motion I would choose grip shift however the circumstances that he is going to be in do not call for it. coast/shore riding is a different beast.