Make no mistake, the KTMs are race bikes. They're awesome in the woods, and very fast. But they do require regular maintenance, and they don't even pretend to be comfortable.
If you don't mind the entry price and turning wrenches, and you want an amazing dirt bike, definitely get the KTM. If you want a low maintenance (but still very capable) play bike or commuter that you just gas and ride, you may want to stick with with the Yammie.
Disclaimer: I own a 525 EXC, have ridden a 530 EXC and WR250X/R. I haven't ridden the new 500's, but I plan on buying one next year.