Like most have said, there are many choices - three or four styles and two engines. I have two 2008GS's and a 2016RT. Had a 2013 ST-S prior. All ride different. If you want to dip your toe so to speak I wouldn't rule out the 2008GS. They should have had the factory services done on them. In my case I'm 6'5" and have riding pegs and handle bar risers; with an Ultimate seat. Super comfy, not leaned forward. Many 2008's are already set up like this, and you can grab one about $3K - $5K cheaper than the RT model. If you know you will be doing long hauls, then the RT will put you riding comfy off the bat from factory. But it costs a lot more. Some dealers might let you ride one for a day or two (used ones). Many like the F3 model that puts your butt closer to the ground and your legs forward. The F3's have the newer 1330 engine. I don't mind tinkering, so I love my 998cc GS's, keeping an eye on oil levels (super important) and adding mods that help out. The 998cc's like to sputter oil up into the air breather and burn some oil.
The main difference I see performance wise is the 998cc is a zippier, more responsive engine great for the curves. A small twist on the grip and you are off like a rocket. A custom muffler on the VTwin sounds awesome too

The 1330 has torque all day, you have to roll on the throttle more. I can definitely feel the power of the 1330, rarely changing gears in hilly areas. But then in the twisties I like the GS's sport frame. We took a long trip to Maggie Valley and the RT was so comfy. The GS, even with the added stuff I probably would made a lot more stops to stretch out, which I don't mind. The ST model is an in between, sitting up more and has saddle bags and was nice on longer trips. I don't own an F3 yet, that's next on the list. But sitting on them being tall it feels great. I use my GS for day or overnight trips & daily commuter. Long trips always the RT, when I can get it from my wife!
Lots to think about, but try to get your hands on one for a day. Try out what you can. I'd start with what you plan on doing and go from there - long hauls, day trips, etc. and your budget.