On the KTM you get USD forks, hydraulic clutch, and 2 to 3 more HP over the KX and better rear suspension. You can also get more HP out of the KTM vs. the KX once you start modifying it.
We have had very good luck with our '01 and '02 KTM 65's. The bike seems to handle better and definitely jumps better than a KX. With the new ergos introduced on the '02, the bike definitely turns a lot better.
We have not any problems with reliability with the KTM 65's since our 1st one in 1999. While we have seen many KX65's loose cranks. With a proper maintenance schedule, the KTM's are very reliable and durable.
Getting parts for the KTM have been no worse than getting parts for our RM85. It all depends on the dealer's willingness to stock parts.
However, the KX seems to fit a smaller rider a bit better. Plus the Kawaski has a good contingency program.