I will tell you what everyone else told me when I threw out the idea of doing a CR500. Tons of fun, but very hard to go fast on. The 250 two stroke, forget about it. Way too unpredictable power delivery to ever feel confident on.
Get a thumper, they put the power down much better and smoother than any two stroke. Your lap times on a four stroke will be much better than on a two stroke. If you are out for just aome kicks, then maybe a CR5 would be great, but if you have any fancies on doing any competetive racing, you must choose a thumper.
Don't worry, we will be here to support you no matter which bike you choose.