The EXC and XC-W are essentially the same bike. KTM changed their model designation for 2006. They also don't come with lights for 2006. The EXC/XC-W have a wide ratio transmission and slightly softer suspension while the XC (formally called the MXC) has a closed ratio transmission (like a motocross bike) and stiffer suspension. I think besides that there's no difference between the XC-W and XC. I would really recommend getting a 200 over the 125. I absolutely love my 200. It has almost the power of a 250 in the midrange and up but it is a lot more controllable in the tight stuff. Good luck bike shopping, I'm sure you'll be happy no matter what you get.
Oh yeah, while I race offroad (harescrambles and cross-country), I also play ride on the motocross track and do lots of trail and fireroad riding with my dad. My bike works fine for everyday riding, but gas range is a little short compared to my dad's XL250 and XR200R. I can still get around 60-80 miles out of a tank just riding. Not sure how far I can go at race pace as I usually pit part way through the race to be safe.