I would imagine YZ parts would be much easier to find, considering they still manufacture the same bike they did five years ago. You might run into the infamous seven-year barrier, where a manufacturer refuses to believe that any of their engineering has survived in working order, and parts availability starts to fall sharply. I should say, though, KTM's reputation is the worst of them all in this regard. If I were shopping for a modern (21st century) 125 two-stroke, I would start with TM or Husky, and maybe go with a YZ depending on whether the European machines are hard to come by in my area at the time, or prohibitively expensive, as well as parts availability, etc. I'd do motocross in a Cadillac Coupe DeVille before buying a Honda - but that's just my little boycott.