like many of the previous responses, my experience is only with gt. 2up the gt is ridiculously more comfortable and nothing short of luxurious in comparison to the sbk (i rode a 748 and a 749 previous to my gt). in my opinion the sbk barely offers 2up, it's more set up for taking wifey or the gf for a quick lap (and frankly that's probably the way a sbk should be set up).
all that being said, be prepared for a major sacrifice in riding style in general/when riding alone. the gt is much different ride than the sbk. dont get me wrong -- i do like the gt -- but it turns like a dog (stock setup, in comparison to the sbk) and the wind sheer is tremendous at 90+ mph. i know you can mod to improve turning (just got mine, havent made any mods yet) and obviously wind screens exist to improve the sheer (perhaps at the expense of looks).
basically, my advice is to get the gt because you like the gt, riding style, looks, and all. it's still a duc, and myself coming off of a sbk still find it to be a thrill to ride. if you are more into sbk performance and only the occasional 2up, maybe a ST or SS would be a better route...