Hey there, I'm originally from West Seattle and I currently have an 07' R1200GS and an 06' K1200LT.
I just recently purchased the GS and have been riding it for the last 3 weeks until I dropped it off yesterday for its first service. I absolutely love this bike and its character.
There really are a lot of factors that come to play for picking a bike. Your size and inseam for instance. How often you plan on going two up. What kind of distance do you plan on riding. I really wanted a fellow member's RT but in the end decided it was too much like my LT and the timing wasn't right but in the end I couldn't resist and instead picked up a new GS because it was so much different and in turn complimented the other.
Riding 2: consider the:
LT, RT, or GS-Adventure, in that order for any sort of real long distance stuff.
Solo riding: any are good but I would tend to go with a lighter R bikes or maybe the new K1200GT depending on other factors such as comfort, terrain I'm riding, and distances.
For longer legs, the GS and GS-Adventure are easier on people with longer legs. The K bikes tend to bring your knees up more, although you can lower the pegs and/or add floorboards on some. If you have bad knees take a hard look at the GS and GSA. I have a really bad right knee and the R bikes are a joy for me.
Long distance two up riding: The LT is king for this, heavy but true and fun in the twisties and a real joy. The RT is a great bike also, although not quite as much room for your passenger's legs with the luggage and much lighter than the LT which is great for nimbleness and ease of getting around, but can make you more easily punished by wind gusts than the bigger LT. If I road solo more often I would probably have an RT or GT and GS or GSA.
Weight: the R bikes are lighter and easier to manuever in and out of "my" garage and such and is a real joy for tooling around in the twisties. The LT is really heavy but suprisingly nimble on the road which also makes is a joy when moving, but definitely is a lot more to handle at lower speeds or trying to get out of tight spaces.
Terrain: The GS and GSA can take you almost anywhere and for me that raises the fun factor. I can tool off down dirt roads where the K bikes would scare the Yoohoo out of me. I would be cautious about taking them too far off the beaten path, consider a lighter dirt bike for these things, but there isn't much they can't handle such as creek crossing, dirt roads, packed areas, fire roads, but are easpecially fun along the twisted tarmacs.
Protection: The LT is again king, especially for two up. The RT close second. The GSA offers nice protection actually and the GS is decent but could at times use more. In the summer heat though I love it. Being 6'2 I wish I had the adventure windshield. I almost bought a GSA because after throwing on a larger windshield and protection onto the GS you've basically paid for a GSA, but after taking a hard look at the kind of riding I wanted to do I felt the GS was ideal because I like the paint better, I prefer the lighter weight of the GS, and for the riding I intend to use it for it's perfect.
Power: They all have plenty. The GT has some sick wow factor that puts a smile on my face, but then again so does my GS. The LT has plenty of giddy-up although on an incline with a heavy load it could use a little more torque but it's nothing a little extra throttle won't cure.
For solo riding: I personally like the R bikes, hmmm no the K, hmmm get em both like me cause I just couldn't decide. They all have strengths and weaknesses.
My GS is so much fun, especially after making a couple changes such as swapping the exhaust out for a different growl. Light and nimble and a fun go anywhere ride. My LT is great going long distances and carrying my SO in comfort and keeping her feeling secure. She misses the wind protection and back support on the GS although I am working on fixing some of these things for her. We test rode the GW and H-UC, and the RT and many many others and she kept going back to the LT so I bought it for her. If not for her I suspect I would have bought myself an RT and a GS/GSA because they compliment each other well in my opinion.
In the end you have to factor this stuff in and test some out and you'll have to decide one way or another. Not easy as they're all great bikes. It just depends on how they fit your body and your riding style. I hope I didn't confuse you. Got rent some and have fun!