I'd go for the YZ. I took a 2004 YZ250 for a ride on Tuesday night and must say I was really impressed. The guy had engine work done to make it into a better woods bike (FMF gnarly pipe, flywheel weight and porting) but was otherwise stock (suspension, gearing, etc). The bike pulled hard, yet manageable, handled well and was an excellent all round bike. With minor modifications (steering damper, suspension revalve, higher bars) I felt I could race the bike no problem.
So, I'd lean towards the YZ. There's a reason why there's quite a few YZ250s at the races. Plus it's cheap and easy to maintain.