It would help to mention your skill level, & intended use.
I have one of each, and am quite familiar with the SoCa dez.
Assuming you're moderately skilled (not a beginner), and like to kick it up a notch or three: RMX hands down. Why? It comes with superior suspension, and packs more firepower out of the box. Although for putt-putt exploring it's difficult to find anything better suited than a KDX - even better than my old XR250R.
The RMX was concieved in the late '80s as a race bike - being tamer than an MXer, it can be play ridden. The KDX was made for play riding, and can be raced (I wouldn't; at least stock). They overlap a bit, though they are better left to their respective intentions.
E-mail me your snail-mail address and I will send you a photocopy of a KDX vs. RMX article. hit-n-miss@(remove to send)DSLextreme.com.