Actually, when dealing with FRS and GMRS radios, the main factors determining link distance are height and obstructions, not power and not antenna gain (which is closely related to antenna boom length for beams or antenna length for monopoles). For example, if you take a 0.5W FRS radio up on top of a mountain and someone else does the same thing 20 miles away on a similar height mountain, that signal may be quite good in both directions if the conditions are right. In some cases it may go 50 or more miles under excellent conditions.
Conversely, in the congested city where there are very tall buildings blocking the signal, it wouldnt matter if you used 50 watts because it would be blocked by tall buildings and bounce around like a pinball. You might not even get a 1 mile radius there reliably even with 50 watts.
The attenuation from buildings and other obstructions can be much more than any gain of a high gain antenna or a power amplifier. That is, if your low to medium gain antenna is low to the ground, you would probably be better off just raising it rather than upgrading to a higher gain antenna. Of course if raising the antenna is not possible, then a higher gain antenna is worth a try.
If you just want an off the shelf UHF radio, the 5 watt Midlands are pretty good. I think you can select low, med, high on those which are 1W, 3W, and 5W respectively. The antenna length is also a fairly good indicator of performance you can expect. Usually cheapie FRS/GMRS radios have very short “stub” antennas whereas the better models are a little longer. The closer the antenna is to 1/4WL on a handheld unit, more likely the better the performance will be. For FRS/GMRS that is about 6.5 inches and for MURS it is about 19 inches.
If you want significantly longer range than a rubber duck can provide and have a GMRS license, you can attach a 3 element beam to a short aluminum pole (enough to reach a few feet over your head) and rig it up so you can carry it in a backpack. The UHF beams for those frequencies are small enough to do that. You should get about a 4x effective power increase so if you use a 5W radio, it will perform more like a 20W radio and have much better receive too (assuming you point it in the proper general direction).
To answer your question, usually the GMRS radios rated with the highest advertised range perform best. That is, it is fair to assume that a 30 mile radio will perform better than a 10 mile radio. As I stated previously, Midland models seem to work pretty well I have tested them. UHF is great because the antennas are small and dont usually get in the way like the old 3 and 4 foot long telescoping CB antennas (I still have some of those).
I hope this all helps.