I'd go for the 250 for a while. Going from no bike to 600cc in the time it takes to get your direct access might be too much. As Dirty DT says, it only goes as fast as you twist the throttle, but a small blip of a 600cc throttle is a lot more power than a small blip of a 125 throttle. You might want to get a bit more road wise before you take on a 180mph sports bike.