Depending on the specific motorhomes, a gas class A could be faster over the mountains than a diesel pusher. It mostly comes down to power at the wheels vs. weight. The gas engine will be running at a higher rpm and so making more noise--and you'll hear it more with a front engine than a rear engine--but there's no doubt it will get you where you're going.
I would start by thinking about what specific things are important to you in terms of how you use the motorhome and where you're thinking of going. If quiet driving is especially important, then a rear engine is a big help. If camping in small out-of-the-way public campgrounds is important, then a smaller rig overall (preferably with decent ground clearance) would be a benefit over a larger, lower one. Think, too, about what sort of interior features you would like: decent counter space for cooking? An oven you can use when not hooked up to electric power (or running the generator)? A bed that you can access from both sides (rather than crawling over a partner)? A shower or bathroom with lots of elbow room? A good spot for a cat's litter box? All of these and others may be tradeoffs to consider.
While renting for a long trip hardly makes sense, it may be money well spent to rent an RV for a long weekend or even a week to get some idea of what it's like to use one and have some hint on features and layout things that work and that don't work for you. This is particularly true if you don't have any experience with one already.
