I too bought a yz250 this past summer. I put a few mods on it and realized that the thing just tires me out so much and I can't use all the power on a track...for fun riding on trails it was ok, but when I had to punch it around every corner to clear doubles, it was a handfull of power...
I started riding my dads yz250f and that thing was awesome! I could use all the power and hit all the jumps that I could on my yz250...except that I actually was faster on the 250f if I was riding multiple laps. So I decided I wanted a 450 four stroke (I can ride the 250f whenever I want). I did tons of research and thought that I wanted the CRF or SX 450...and last on my list was the yz450f...After riding an 02CRF,03CRF,04CRF, and an 03YZF---I took the 03yz450f. All stock except for 4oz flywheel weight and Tag clamp/bars, one larger tooth sprocket....the thing is so nice! After taking it to the track I was so glad that I stuck with Yamaha on this one. The ergonomics were familiar and I know how to work on them. I thought it was going to have a "violent" hit, but I experienced no such thing. The 4 speed tranny is really nice too. Forks are wimpy though. The YZ has more space than the CRF.
OK...as far as 250f's and 450f's go....the 450 power is managed very easy, it will be easy for you to ride, especially if you can handle the yz250. Go with the 450, you will be happy with it. The power delivery is so smooth compared to a 2 stroke it will make your riding really easy, no more clutch work, just lug around a corner in 2nd or 3rd and roll on the gas and overshoot those doubles! Oh yeah, with the 450 the power is so smooth that the hardes part for me to get used to is not overshooting jumps, you don't really realize how fast you are going. You will soon learn to whip and fly low. The 250f I often top out on power and need more...but it is a really fun bike too. Try riding a few. I personally like the yzf over the crf. sorry this was so long, but I hope it helps...I know what it is like trying to figure out what bike to get..its a big investment.
see ya