Finding aftermarket and replacement parts for the YZ will be quite a bit easier. As for the KTM, here is something to think about:
I read a very long and detailed article about the KTM 250sx vs. the 250xc and the panel of multiple professional as well as amateur riders concluded that unless you are going to be riding 90% motocross they all preferred the 250 XC over the 250 SX. The sx is very specific to riding in mx situations. The xc, however, might have been slightly heavier but was much more comfortable, RELIABLE/DURABLE (that's huge), and all around a better performer in every discipline other than Motocross. The xc performed very well on the mx track but with the slight addition in weight and softer suspension set up it was not as competitive on the track. Every single rider stated that if they were not trying to race motocross competitively that they would all choose to have the xc over the sx, so if I were you and I decided to go with the KTM over the YZ, then I'd absolutely go with the xc over the sx. Hope that helps you out a bit...