In my experience I would say go for a 600 to start with. They are much more forgiving and will allow you to make little errors of judgement that we all make when learning to ride. A 1000cc bike will turn your little errors into massive problems very quickly.

For example open the throttle too quickly coming off a roundabout and it will either lose the back end or lift the front at which point to head straight across the road with no steering. Maybe a bit extreme and I know people say they only go as fast as you make them but it's so easily done with 140bhp at the back wheel.

At the end of the day it's your choice and only you will know if you can exercise enough restraint keep it under control but IMO a 600 will teach you more about riding than a 1000 and TBH you can't even use the full potential of a 600 on the roads these days.

2p