Im in the exact same situation as you my friend. My top two choices are the kx and yz. Im currently leaning towards the 2015 yz250f because of all the changes they did to the bike in 2014. Reward slanted motor with rear facing exhaust better for centralized mass which is better for handling. Shorty exhaust - ties into centralized mass. Gotta love the air box location infront of the seat with the sleek hidden gas cap. Excel rims which are legit excel rims, i contacted excel and asked if they just got paid to have the logos on the rims. Yamaha motors are more durable than kawis. I just sold my 2012 kx250f, great bike, rocket of a motor but the bike started to deteriorate quickly. 90 racing hours, new top end at 75.
The 2015 250f shoot out results will be your best bet to decide on what bike is better for you. The yz250f's will win more championships in 2015. If kawi redesigned their chassis to make it handle better in the corners then id buy it, but they havnt changed the chassis in years. Im a big kawi fan so its going to be hard for me to change but there are way more pros than cons with the new yamahas.