CB's are great!  I advocate them, promote them and think anybody going 4wheeling or trail riding needs to have one.  The more power the better.
But....
I also think that a GRMS radio is, ummm, better.  Here's my reasons:
Better range
portable
more channels
less expensive
no wiring, no antenna
Get them both!  Get a CB and get a couple hand held GRMS radios...  I own two really good Motorola's and I bought a 4 pack of some cheapy, cheapies...  I've gone with quite a few different groups and levels of 4 wheelers and you'd be surprised how many people don't have 2-way communications.  Well I always bring extra's along, and it's amazing how much fun it is to wheel with communication.