Great reading your stuff as usual. You're eyeing up a new 250F, and here I am being inspired by your old CR, and am looking to find my first "real" mx bike; a 1986 Yamaha YZ125. I imagine it'd be fun racing vintage on it, though I recall the bike totally sucking then, but then again I was racing against guys on bikes 13 years newer than mine.
I had a chance to hop back onto a 125 a few weeks ago, and got to compare it back to back with a crf250x, and a wr450f. I forgot how much more work it was to carry the same speed on a 125. Granted, it was an RM125, known for piss poor bottom end, and the tires were stock from 2003, but needless to say, the power on them is not very forgiving. If you're currently racing guys & placing well on a 125, I'd almost guarantee that you will blow them away if you hop onto a 250F. They are just easier to ride. You can botch a turn, and there will at least be enough torque to fix your mistake. You can still ride them like a 125 2T, just don't bounce off the rev limiter like Barcia off every jump.
Regarding FI\Carbs.......I hate carburetors with a passion. If I'm dropping the coin on a new bike, there is 0 chance that it will have a POS carburetor on it. It's antiquated technology that Yamaha has been peddling for far too long. They must have a ton of stock of those carbs floating around, because there is 0 reason to choose them over going Fuel Injection. The paranoia about FI failing is so silly that it hurts. Maybe we can convince car manufacturers to go back to Carburetors & points\condenser ignitions while we're at it......yah know...in case things fail when we're in the middle of nowhere or something
So long story short......Roost the 125's for this year. Wait for 2014 YZ250F with Fuel Injection, and in the meantime save up for spare valves & such.