Here is your dilemma. In your first post you say:
"will it take the big hits as well as a KX, or close even?”
ANSWER: No
“I loved the KX power band”
ANSWER: The KDX has a totally different power delivery. Lots of power down low with a mild hit up top. That said, they can be modified to have good low end power and a great hit at higher rpm’s (see my signature for modifications).
“How do the suspensions compare?.. I like air…”
ANSWER: You might like air but the KDX’s dampening rod style, conventional forks don’t. With modifications the KDX forks can work OK in both woods and on the track, but they will never be very happy about track duty. If you like to be competitive on the track, a KDX will let you down.
So you have a few choices
(1) Buy a KDX and modify it. I have spent over $2,000 modifying my KDX to get it to the point where I like it in all conditions.
(2) Buy a KX and add an E-line (or similar) lighting kit. The e-line enduro kits sell for over $600 but they include a headlight, taillight, lighting coil (100 watts x2), wire harness, hydraulic brake light switch and three plus gallon fuel tank.
(3) Buy a KDX and add a KX front end. Some on this list have done this and have posted “how-to” articles. Do a search for information.