the best way forward is to decide what you WANT from your bike, then decide which type of bike bests suits the bill. no use buying a pocket rocket if you're commuting all the time. or a commuter if you want to ride the twisties with your m8s every sunday.
b4 you do anything daft like actually buying tha 1st shiney bike you see, check the insurance-it makes me cry the amount of money young uns pay out in ins these days.
if you go for a smaller cc bike, i would steer clear of "unusual/rare" types, (harder to sell when you want to trade up)
the smaller bike will let you learn your craft, while building up some ncb.
when you do your das course, if you started on a 125, then moved up to a 500 i would go for a mid range bike 1st (its the schools way of saying your safe, but not that confident at 1st)
you also have to remember that while a cbr600 & the like is only 100cc more than the training school bike, its a LOT more powerfull, sharper handling, sharper brakes etc.
last bit of advice, once you pass the test, you're only just starting out on the journey, you havent just finished OK? i've been riding 28 years & i'm still learning(see my profile for the boring bits)
last bit -honest

ride to your own level, dont give in to peer presure & try to keep up with others-thats where the accidents start to happen, as another well qualified member says "better to arrive 2 mins late in this life, than 20 years early in the next"
HTH & enjoy the journey.