In my experience, the modern SS is a very poor choice if you have a pillion rider in mind. Power is okay(ish) but ergos are a pain for the unfortunate victim and restrictive for the pilot. I've never taken a passenger on a 749 but I would guess that would be even less suitable.

Going from a ST4 to either of these bikes with passenger accommodations as part of the equation is going to leave you wanting.

Thought about an ST3?