I've owned both and used both.
In identical installs, the Rangemaster gets better range. That's because if you know what you're doing you can tune the Rangemaster for EXACTLY 100mw input to the final stage. You can't do that with the ProCaster, so it has a bit less for margin of error (so as to not go over). However, the ProCaster is easier to tune.
With external audio processing, in my opinion the Rangemaster has better audio quality. However, the ProCaster has built-in audio processing, and it's no slouch in the audio quality department either.
It really all depends on what you want. If you're willing to put the extra time and learning into the Rangemaster, it's probably a superior transmitter, and will give you a bit more range, all other things being equal. On the other hand, the ProCaster is about as plug and play as you can get, and is still a very high quality unit.
In terms of reliability, I've found the Rangemaster better. Maybe I'm a one-of, but I've had several ProCasters go bad after a few years, whereas the Rangemaster just kept on ticking for years and years.
these particular rangemasters have a custom modulator section that is more high quality and superior to the typical rangmeaster opamp and small audio transformer section. keith has told me the custom modulator section since it is audio circuits only and not the rf circuits does not void certification.
I myself am thinking of the Talking House or IAM unit currently i'm running tests on FM using a CCrane 2.My preference would be to broadcast on FM but the frequencies are sparse (Over a half dozen translators)and of course the possible chance of an FCC visit.AM frequencies are more prevalent in my area (Lexington,KY)so i'm leaning more in that area.