HTs on the amateur bands are basically limited to VHF (6m, 2m, 1.25m) and UHF (70cm), so not really any different than GMRS (UHF). Mode (FM) and power levels are basically the same as well. It basically comes down to the things you already mentioned, like terrain, as well as antennas, availability of repeaters, etc. In my area there are a handful of GMRS repeaters, but there is at least one ham repeater in almost every city in my county alone. YMMV