Almost everyone that has a diesel will tell you its the only way to go. Diesel owners are very loyal.
For those of us who have never had a diesel, we will tell you that a gasser is just fine. I don't have issues with heat, noise or ride...........that I know of. I say "that I know of" because I have never driven a DP to know what I amy or may not be missing. My point is that unless your doing a lot of traveling or spending your time in the mountains, a gasser will serv you well. Just make sure the AC on that 2000 is up to code and working strong.
3 months is plenty of time to know if you like your C or not. a 28' C has the advantage of being more nimble and fitting into smaller spaces. The disadvantage is the room in it. It really is designed to get you to and from, provide you a place to sleep and get clean and that's about it. There is nothing wrong with that if that is what you want/need. If you want to spend more time inside than outside you may want a bigger rig but even a 28' class A would give you the same restrictions as the class C.
The 31' Class A will give you a little more room but not enough that would be worth taking a loss on your current class C. If you can do it without taking a loss then it may be an OK move. If not you may want to think about going a little bit bigger (if you plan to spend your time inside and not outside).
My recommendation would be to go to a good size dealer and drive a few so you can see what you like. Who knows there might be a bigger class C that you fall in love with.