Ideally you want both.



But failing that the biggest difference that you'll feel every time you steer of centre will come from the rear sway bar.



The two mods that create the most appreciable difference in driving dynamics (with the exception of aftermarket wheels and suspension) are a Cold Air Intake (any form of improved induction) and Rear Sway bar.



The change/improvement by the addition of a strut tower bar is much more subtle and will not be notable to a lot of drivers.