I can't envisage a scenario where you're any less likely to kill yourself on a 600SS than a 750SS. Neither have the power to light up the back end by mistake (we wish!) and they're nearly identical in terms of handling/braking/etc.
I have to agree with saj288, the quicker you get on a bike you're comfortable with, and start focussing on all aspects of training, the better.
Good luck and enjoy.
Ali.B