I have owned 4 gas based motorhomes and going up steep hills, or accelerating onto a interstate you will of course hear the engine that is just below and to the right of your feet. The one I just sold, a 2000 Hurricane 30Q with a Chevy Vortec engine, was however quite quiet under normal driving conditions. They will never be as quiet as a DP where the engine is in the back some 30 feet away.
I'd match the size, maintenance costs, amenities, etc to how you plan to use the thing. If you plan to use the unit full time or perhaps weeks at a time, then the bigger the better. Weekends, summer vacations, stay at state parks, then you have more options.
I can tell you that if you start looking at a decked out 40 ft DP, then step into a 30 ft gasser testosterone, ego, or DW will make the decision.