I went from a tricked out KDX to a KTM. I test rode the 200's and 300's before deciding on the 300. Personally I thought the KTM 200 and KDX motors were Very similar But KTM obviously has the edge on components and such. The 300 is like a KDX, just a Lot more. A Lot more low end, and gobs more top end, if you ever get to it. I just lug around everywhere on the 300, always short shifting and Love it.
I still miss my KDX sometimes. Especially when something breaks. Parts are kinda pricey. But the breaks, clutch, suspension, etc all make up for it. If you're looking for more motor than the KDX had, I would definitely go with a 250, or even the 300. They're All great choices though..