God (I bet you like the sound of that),
I went with the OEM sprockets. They were about the same price as aftermarket and seem to be of good quality. KTM specifies steel front and alum rear. JT (and probably others) make a steel rear that is a little cheaper ($30 vs. $50). A friend of mine went this route, and while the thing should last until the next ice age, it was amazingly heavy - it didn't even have a single lightening hold drilled in it. He ended up drilling some holes on his own.
You didn't ask, but I'm running the stock 42 rear and the larger 17 front to keep the highway revs down. I've got a spare front 16, but I've felt little need for lower gearing off-road, even on a fair amount of single-track (e.g., Belfair).