I find it really interesting that your choices are between a cruiser and a sport bike.
The Shadow 600 really isn’t that much more powerful than the Vulcan 500. It may be a bit bigger and harder to wield though, I’ve never sat on either. But it and the S40 (a 650 single cylinder cruiser) are all fine beginer bikes too.
I recently saw an experienced rider on an SV650 in a controlled environment, and to be honest, I think I’d much rather learn on something lighter. The guys on lighter bikes, with far less experience, were having an easier time with bike handling. If you want ot learn how to ride a sport bike really well, start with something light that you can really toss around. It’ll make you a much better rider in the end. And going by the smiles on people’s faces, it wasn’t any less fun.