I've had the KX for a couple of weeks now and have ridden it several times so I thought I might update this post. I notice a couple of things:
1) the KX is shorter than the KDX. I don't know if the guy I bought it from (an AX racer) lowered it, but it's noticeably shorter than the KDX. Being 5' 5", this is good for me. The biggest difference is from the "cup" of the seat on back- the KDX rises and the KX stays flat.
2) the KX is MUCH lighter. This is immediately noticeable when pushing the bikes. I can almost toss the KX on the trailer and the KDX requires some grunting and digging in to push it up the ramp.
3) surprisingly, the KX power is very controllable. It almost never wheelies on me and the KDX will easily wheelie coming out of turns. The KX will get from point A/B quicker, but it doesn't want to loft the front wheel on the way.
4) the KX suspension (modified) is excellent, but very stiff (AX settings, still). The KDX suspension (also modified) is surprisingly good in comparison. The KDX moves more in the first couple of inches, but is much more resistant to bottoming that I would have thought. Jumping the same jumps and going about the same distance, the KX is like jumping onto a trampoline and the KDX is like jumping onto a pillow. Over the whoops, I appreciate the stiff front end on the KX, but everywhere else the KDX is not bad at all.
5) Jumping- I was really suprised to find that the KDX flies just about the same distance as the KX. In fact, it may go farther for the same distance. This means, of course, that I can hit jumps faster on the KX, which is probably the idea, but it does take some getting used to. My first reaction is "Oh no!!! I'm going to WAY over-jump!!!", but no....the KX flies just about the right distance. However, after riding the KX I took the KDX out on the track and nearly looped it on the first steep jump! The bottom end is so much stronger that it fooled me. After I adjusted it was fine, but jumping definitely takes a minute to re-adapt to each bike. In the air, the KX is MUCH more sensitive to throttle chopping- it definitely likes to hit a jump under power. There's a set of doubles on this track that, as long as I have the speed, I can yank the KX over and I was scared to even try it on the KDX.
6) I noticed a big difference in handling. I used my riding buddy on his XR250 as a "pace car" around the track, with me following him and adapting my lines according to which ones he picked. I found that with the KX I can easily change lines mid-turn. If he took the inside, I could easily rail the outside and pass him. If he came in tight and went out wide, I could easily change lines and cut inside his wide line. Following him, I felt like a fighter jet following a Cessna. When I switched to the KDX, the first thing I noticed (after nearly looping the bike....see #5 above) was that the KDX "wallows" in the turns. I definitely could NOT whip my lines mid-turn. Whether this is because of the higher COG on the KDX, the softer front end, different fork angle, or just the inability to move on the seat much, I don't know, but it is the one place where the KDX was really lacking. I'm going to work on that a bit. It's funny because I never noticed it on the KDX until riding the two bikes back to back like that. Instead of feeling like a fighter jet, I felt more like another Cessna and it was definitely a bit harder to pass the XR.
Overall, I'm learning a lot and having a lot of fun on the KX but the KDX is still an awfully good bike. Given only one, I'd probably keep the KDX. Makes me wonder...just a little bit...if a KX250F would combine the best traits of both bikes....hmmm.....