I've owned a 2007 KTM 250SX - much better suspension and motor than the CR250, nice that you can tune the motor by ignition setting and power valve spring.
Later I had a 2009 KTM 450SX - awesome very easy bike to ride, very poweful and controllable motor, feels heavy but has elec start which is wonderful
Now I've got a 2012 KTM 150SX - best bike I've ever owned, more powerful than a 250F, ultra light weight, good suspension. Not the most powerful bike on the track...just the most fun
I have tried a KTM 300 and found it too had too much of a trail bike power delivery for MX use, likewise the KTM 350SXF had slightly more power than the KTM 250SXF but felt like it weighed the same as my 450SXF.
Really depends what type of riding you do - for trails buy the KTM 300 - it is the classic trail bike. For MX If you like your CR250 then buy a KTM 250SX. If you want a 4 stroke then go for the 450SX, and if you can get your hands on one then go for the Dungey replica it is excellent it feels much smaller and lighter than the current 450SXF and it turns really well