John wrote:
Can anyone tell me what the difference in performance would be between
(handhelds): a 5w GMRS vs. a 5w Marine VHF in mountain terrain
(ignoring legalities.)

Performance? Do you mean range? If so, the answer is: It depends. The
GMRS radios operate on UHF (450 Mhz band) and the marine radios operate
on VHF (150 Mhz band). The path loss on line of sight communications
will be greater on UHF, but improved receiver sensitivity and
easier-to-achieve antenna gain can offset that. The specifications on
the individual receivers in question are probably more important
determinates of range.

Worry more about the legaliities and less about performance and you'll
be OK.