Both are two-stoke bikes, so there's plenty of power on either one. The KX is a moto-cross bike so it has a stiffer more advanced suspension, while the KDX is an "enduro" that has a plusher more comfortable suspension geared at trail and woods riders. Both will do wheelies in 2nd and 3rd gear with a twist of the wrist.:scream: The KDX is more user friendly and requires a little less maintenance, like piston and ring replacements, etc. For a beginning rider, the KDX is the way to go, unless you will be spending the vast majority of your time on the track. The neat thing about a KDX, as you'll see all over this board, is it's ability to "grow" with the rider with a few simple aftermarket performance parts. It's much easier to grow a KDX than dummy down a KX to make it more friendly.
The KDX is more user friendly, more forgiving when making mistakes than the KX. If you'r going to be riding nothing but trails, get the KDX. But if you'r going to get into riding some motocross tracks, riding half motocross half trail or want to be doing ALOT of jumping once you get more use to riding (and you will) you might consider a KX. But you can get on a good early to mid 90's KDX for around a grand, then once you get to where you have raised your level you could upgrade to a KX and atleast get what you gave for the KDX. That's if it's not trashed from learning.;)