Really depends on what you wish to gain from street riding?
I have had many different styles of bike and also come from motox, my favourite street bike was a bandit (street fightered with renthol bars) which handled brilliantly, I know this is a cbr forum so that's all i'll say on that.
But for speed, comfort, style etc i'd go with the 600 cbr. You can obviously ride but just be aware that street riding has so many more hazards than off road.
Personally I would go straight to the 600 and just really take it easy to begin with to get used to cornering, speed and acceleration.

 

Thanks for all of the input everybody. I'm thinking a 600 it is. I know street and dirt riding are going to be very different with worrying about other cars etc. But I think I will be able to handle a 600. I'd like to think I'm a pretty level headed guy, so I'm not the type to go get on the bike and see how far I can push it right away. I'm very cautious about slowly learning mine and a bikes limits. I just can say for sure that a 250 would just bore me, I see tiny little girls riding them around and since I'm a 220 pound guy I think a 250 would just feel like a kids toy. Thanks again for the input.