F KTM. :stirpot:
Either way, if you weigh more than 160 you'll be unhappy with either of those IMO. I rode a DRZ400 for 3-4 years and really liked it. Inherited my dad's 07 KLX250 when he passed away and it was incredibly underpowered. Sold that after 2-3 months and bought an RMZ450. I weigh just under 200 and that KLX250 felt like a moped.
If you're stuck on one of those two, I'd go with the Yamaha.
I still think the DRZ400 is the best DS made by any of the big 4. KTMs, Husqvarna, Aprilia, Gas Gas, Husaberg, etc. are sweet, but $$$$$$$ and parts availability isn't as good.