I did exactly what you are contemplating back in April of this year. I had owned a 2002 R1150GS (the best year they were made, btw) and moved 'up' to a 2004 K1200GT. Now, keep in mind, like prolly 90% of GS owners, I never took my GS off-road on any more than gravel backroads, that being said my impressions of one bike to the next are:
* The KGT takes more effort to ride.. steering at speed esp. takes a bit of effort when compared to the 'power-steering-like' feel of the GS.
* The K-bike motor ROCKS!
* The KGT will always feel heavier underway than the GS did.
* You will get used to the change in ergonomics pretty quickly, at least I did.
* The K-bike motor really ROCKS!
* The GS has more between fill-ups range than the KGT.
* Passenger accomodations are way better on the KGT, esp. w/ a backrest installed.
* The K-bike motor really, really ROCKS!
* The GS is easier to maintain by yourself in your garage (less plastic, less complicated)
* The KGT needs less maintainence overall (no TB balancing, no surging issues, etc)
* Did I mention that the K-bike motor really ROCKS?!!
Seriously, though if your type of riding is on a mix of open backroads, with sweeping corners and connecting straightaways, and fast four-lane transits where you need to be able to simply dominate surrounding traffic, then the K is for you. If you prefer tight, super-twisty chicanes and always keep it under 80 or so, then I'd say you ought to stick with the GS... Of course, you could always own both!