The big benefit to hard mounting the GMRS radio verses the hand held is you get much better antenna options. Most hand held antennas are pretty dinky. Nothing wrong with them (I use hand helds all the time when driving with people and we can communicate car to car easy, just without as much range as something more powerful), but you get a lot more capability with a better antenna. There isn't much functional difference between some of the higher wattages (a MXT275 at 15 watts is not 66% worse than a 50Watt MXT575) because obstacles in the line of sight will limit you much more than power. Better antennas help, but it doesn't matter how much power you are pushing when you are terrain masked, you are terrain masked.
In a closed metal car, a handheld is at a disadvantage because the car's structure acts like a faraday cage and can mess with the ability of a hand held to receive and transmit. That's much less of a problem in a soft top (or even hard top) Jeep or Bronco just due to the nature of the construction.
@AcesandEights is exactly right about hand helds on trails- the line of sight is really the limiting factor, but they are going to be great for spotting and communicating with the group. Always a good move to have a couple hand-helds around to give to your spotters or other people in your group. I always clip one to my elderly father in law's belt whenever he goes somewhere with us just in case he gets lost. Cheap insurance.
The other advantage is that generally speaking, a hard mounted radio will be more likely able to use your local repeater network. There are many different hand-helds that can do this too, but almost all the mobile radios can and that's a benefit where I am. Clearly a nice to have not a have to have so YMMV, but a good repeater helps solve that terrain masking problem. this is probably not an issue on a trail ride- you are probably always going to be within LOS (radio wise anyway) so not something to really worry about. Here in NM, there are lots of places with no cell service, but I can hit the repeater network from my car and could probably call for assistance in a pinch.
Finally, and maybe this is just lazy on my part, but a hard mounted radio is just one less thing for me to loose. That's worth something to me.
@jimFish you asked about radio waves in the cabin- anytime you are irradiating anything that's going to be a concern. That being said, I have no problem using a 5 watt handheld right next to my head and in a closed car. I might not be super calm about say attaching an antenna to a hard hat and running a 50 watt radio through it, but that 15 watt MXT 275 is going to be okay. In a Bronco or Jeep, there is an escape route for the wave so there is not going to be a lot of refraction in the cabin. Also, you are only radiating when you are transmitting so thats a pretty small time window. But again if its a concern, there are all kinds of good mounting options outside of the bronco- Rear mount, lip mount on the hood, cowl, or even on the trail sights.
