I'm lacking in both engine and skills. I did the STXC stuff this year, and it did help a lot with the skills. Plus, it was a ton of fun. I will say, I don't think it helped my skills as much as the CX clinics I went to at Alpenrose -- we practiced a lot of off-camber and 180 degree turns, which is much more typical of what you see in CX than the average STXC race. Definitely do the STXC though. It's worth doing just for its own sake. But I digress....
I did the STXC races this year on a full-rigid 29er with drop bars and 29x2.1 tires. I figured this would be sort of like a CX bike, but a bit more forgiving. It wasn't a bad choice for the races at PIR, but when I used it at Westview and Sherwood, it beat the crap out of me. The other problem I had was that my 29er weighs about 30 poounds and I don't think the extra cushion of the tires was worth the weight penalty.
I'm still trying to figure out what bike to use for the short track races next year. I've considered putting some 700x42 knobbies on my Cross Check, which would be a lot like what I did last year but with a much lighter bike. I'm also toying with the idea of using an '89 Rockhopper I've been tinkering with -- I think I can get it down to about 22 pounds with modern parts, but it'll still be fully rigid.
If you're doing this just to build skills for CX, then I think using your CX bike is the way to go. The thing is, if you decide you want to do well in the short track races, a CX bike really isn't the best choice. I'm not sure a rigid mountain bike is much better, but cheap mountain bikes are so heavy.....