Ok this is prolly old now but it may help others with there choice!
The 2 stroke yz250 is an excellent bike that is reasonably easy to look after. The yz 426 is a much more powerfull bike but with that comes the extra bit of maintenance required.
Firstly the 2 stroke you just need to keep your mix the same, change your oil and do a piston and ring kit occasionally! (depending on how much you ride) the yz 250 is very light and a good allround bike! (very easy to start)
YZ 426's Well forget the crap! These babies are hard to start! Require oil changes all the time and require a fair bit of hanging on to when ridden! The issue with starting can be cured tho! They flood easy so keep away from the throttle. Follow the start procedure exactly (nothing like a mate to flood your bike for ya while its on the stand)
"Gearboxes" yz 426's have issues with shift forks being bent which results in an expensive bill! If you can repair this yourself not so bad!
Power wise the yz250 with a paddle only then starts to feel like a 426 in the sand!
426 will put an instant smile on your face when you ride it! Even more so when it starts regularly!
Best mod for one is to replace the exhaust
cam with an 03 auto decompression one! Smooths out the hit by increasing bottom and top power!
I've owned both bikes and had a look inside both internals!
Easiest to fix - YZ250... Easiest to maintain - YZ250... Lightest - YZ250... If broken cheapest to fix YZ 250... Easiest to ride in a straight line YZ426... Biggest jumper YZ426... Biggest smile factor YZ426...Scariest YZ426...
Hell its up to you but once you have had a four stroke its hard to go back!