Opinions and thoughts from a recreational MX rider and 01 250F owner:
The 250F is going to be about the same weight as your KX250.
That really helps with the fatigue factor is very noticable compared to the 400-426. Of course the 426 power delivery can be tiring. I think that it was Dirt Rider that called the new 450F an "arm pump machine" or something similar. Anyway, the weight and power delivery of the 250F will be a noticable change from the 400-426.
You will not get hole shots with the 250F if you race in an age class.
Is winning the race or having fun your first priority?
Winning = 400-426
Fun = 250F
To ride the 250F fast, you won't be able to rely solely on the throttle - you'll have to be smooth and carry your momentum - which may make you into the better rider you said you'd like to be.
Two possible scenairios:
1) You get the 250F. You're 10th coming out of the first corner. You have plenty of energy for the full moto, and pass your way up to 4th or 5th, and finish strong while others fade.
2) You get the hole shot on the 426. After several laps, the power and weight start getting to you, and a couple riders get by you. You fade to 4th or 5th and are whipped buy the end of the race.
You get the same trophy in both cases. How do you feel about the day?
I'm sure that there are plenty of 426 riders that handle the bike and power without any problem, and maybe a 250F's power may leave you feeling flat, after racing a 250 2-stroke. Ride one if you can.