| Get the 750. Treat it with respect and take it easy. It has no more power (in fact considerably less) than a Jap sports 600 or even something like a Fazer, so a bigger engine doesn't always mean more scary.
I've never ridden a 750, but I am reliably informed that they have a lovely smooth engine. If it's anything like my Duke it won't have huge amounts of power, but it will have the lovely flexible power that you can use easily with no fear of it spinning the rear tyre (except in the wet of course). The sort of power that you can use most of the time wiithout it getting you into big trouble.
If you take your time learning to ride it, you'll soon get used to the power. As somebody has already pointed out, it'll only go as fast as you twist the throttle, but it will have all the other characteristics of a bigger bike so I reckon it'd be a good un to learn on. The weight shouldn't be an issue. Like all V twins it'll be heavier than a Jap 600 but not by a huge amount, and unless you're built like an elf you shouldn't find it too heavy.
The Suzuki SV650 is a bike that's often recomended to new riders because it's nice and friendly to ride. Since the Suzuki is a 4 valve engine and the Duke is a 2 valver, they'll be similarly powered bikes (except that the Duke will have more grunt in the mid-range courtesy of it's extra capacity and two valve characteristics). So the 750ss will be similarly user friendly - BUT, it'll be a real Duke  | 
Jools
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