690. You mentioned 50/50 street dirt...hence 690. And there will be things about the 690 that are not perfect, but I reckon you will love it from what you describe. 500 is for dirt. You can link trails with it, but sitting on it while on road for any length of time sucks (though it wheelies forever :0).
Get the 690, then sell the KDX and get a 300XC or EXC....perfect quiver.