the mags arent your best bet when it comes decision time for your next bike, then again, you probably wont be dissapointed with either.
it will come down to what you prefer and what you feel most comfortable on. go sit on a stock KX and YZ at your local dealer( if you have one) and furthermore, you could try to find a buddy who has a KX or YZ that is relatively new. if you want stiffer suspension, more top-end power, and a taller bike, then get a YZ. if you want a more plush suspension setup, more low-end/torque, and a better handling bike, then get the KX.
bottom line, theyre both great bikes, quite possibly the best of the 250 smokers, so i would probably base your opinion on which one is cheaper in your area. both can be modified to fit you, but itll usually maintain that same feel that it came with. good luck with your choice :nod: