Depends on your weight and how serious you are about the MX/s-moto portion of things.
Close friend of mine has a 250x, it's the reason I own a 450x today
. I'd have bought a 250x but the lustre of his was waning when he just got tired of wringing it in the sections of trail that actually let you use the big guns. The 450x has more power then I'll use, but it's very tame compared mannerswise to my 03 Yz450 with about 3k in 'woods conversion' to it. Side by side in any trim I think the 450x would hand the Yz it's ass too, if for no other reason then more tractable linear power delivery.
Weird thing about the 250x is that i was faster on it because it was so easy to flutter around the trails on. With my weight on it (250#) though it picked up well - just had to row the damn shifter alot.
If you want the power of a 2 stroke and manners of an XR - get a CRf450X. If you want to hang out with your family but still have a fun time even though you're just out tooling around - get a 250x. It's really similar to the 1000/600 thing on the street...you can make up for crap riding easier with the big HP but it can get you in over your head doubly fast. OTOH if you like lofting over logs and stuff at any gear, any time - the 250x becomes a bit more of a burden and does require a bit of planning.
Both have E starters and both are kickass
