Sounds like you've got the same dilemma I had. All throughout my time as a learner (and even up to taking my test) I had people telling me to buy a big bike and restrict it rather than buy a 250 etc but I knew that I struggled the last time I rode a heavier bike. The thing that helped me make the decision is knowing what sort of riding I wanted to be doing and inevitably what sort of bike I'd want to get/work up to