If AWD is important to you, have you considered the G37x S? Not familiar with all the trims available in the US, but in Canada, an AWD S version is available (though you'd lose the 6MT).
Is the interior and trunk room of the S4 enough for you? Mind you, I've brought photo gear including small assembled softboxes to photoshoots with my Z...but there wouldn't necessarily be enough room for a passenger in some cases. So, either S4 or G37 should be enough for photo gear...I guess it depends on how much stretching room you want for your passengers. BTW, doesn't the current S4 have a supercharged 3.0L V6...or are you looking at a pre-2009?
Between the two, I would choose the G37.
Good luck with your decision.