I honestly couldn't say about GMRS, other than get one with a removeable antenna. After weighing the options, I went the ham radio route. I have a brick amp and sla battery mounted in an ammo box that allows me to transmit at 30 Watts for a few hours. I think you can go up to 50 Watts with GMRS, but that is only a "land based" station.
The stub antenna on GMRS/FRS units are way less than quarter wavelength, which is bad news for efficiency. What happens is that the input impedance of these stubby antennas is not matched to the output impedance of the transmitter, and a lot of your power gets reflected back into the transmitter. A true quarter wave monopole antenna is better matched and will not reflect as much power. Even better is a j-pole that you make out of TV twin-lead or rigid rod. It is perfectly matched to the transmitter and has a radiation pattern that is more squashed toward the horizon. The Germans invented this class of antennas because they are end-fed and the antenna was just a wire that streamed behind their zeppelin. I torture my students with the analysis of the j-pole.
Here is a design for a rigid GMRS j-pole. If you use twin-lead, you can roll it up when you're not using it. But you have to tie it to a branch when you want to communicate.
http://blog.mecworks.com/articles/2005/02/27/gmrs-j-pole/ The real advantage of ham is the access to many repeaters, as Tim in NV noted. As an example, up near Eckler mountain in the Wenaha Tucannon, at 5 Watts, I could reach the Spout Springs repeater near Tollgate 30 miles away and talk to folks in the Tri-Cities 70 miles away.
There are GMRS repeaters as well, but they are private I think.
