The KDX is a great trail bike, but any bike can be fun if you like it. If you are comfortable on the RM, then by all means keep it.
As for rebuilds, the RM will certainly require more frequent top-ends, but the time interval depends on how hard you ride it, not where you ride it. The engine doesn't know if it is on a trail or a track, or whether you are racing or having fun. The best advice an give you on top-end intervals is to do a compression check afetr break-in, then recheck it every 2 or 3 rides. Once you have lost about 10% of the original compression, it is time for a rebuild. My engine is for all intents and purposes a '90 KX 250, and I generally do a top-end about every 40-50 hours (usually about 5 or 6 rides). I trail ride, and I use the engine to it's fullest whenever possible.