I was being sarcastic when I said 'what are you going to do on the second week after you learn to ride the first week?'
But seriously. If you can get a 600, Id get a 600. Will save you shit load of money down the track, and you will learn on a decent bike.

You will be bored and disapointed on a 125 or 250. I have seen it many times.

It doesn't take long at all to learn how to ride.