I own both a GS-A and a GTL, both have non-stock seats with backrests and windscreens. Both have "highway" pegs to allow me to stretch my legs occasionally, although I rarely use them, and only one at a time, just because I don't like the modified wind flow.
Lets get the issue of the GT vs GTL out of the way first: If the bulk of your riding is secondary roads, solo, and you "fit" (generally means under age 60 and in decent physical condition), the GT may be just the trick for your only bike. Otherwise, GTL.
GS or GS-A: Mine has the Australian electronic cruise control added. VERY expensive, and worth ever penny! Without that, I'd leave this machine, or any other motorcycle without electronic cruise control, out as a everyday long distance cruiser. Once past that, this is primarily a solo machine. A bit awkward getting the passenger on board, although its no problem once they're on. These machines tend to have semi knobby tires, which last pretty well, but tend to make the whole machine vibrate a bit at elevated speeds, which can become a bit tiring, plus the engine has noticeable vibration, although not nearly as noticeable as tire vibration when the tires are new. If you can deal with that, they make a superb long distance machine. Thousand mile days become just a matter of staying awake! The sound system sucks! Oh, wait a minute, it doesn't have one!
I've added sound to hear my radar detector, GPS, and music, works fine, but its surprising how little spare space there is on this giant machine.
GT(L): Has built in cruise control, fantastic braking, well actually, most everything about this bike is fantastic, although I'm not fond of the standard sound system. If it had an easy way to integrate a radar detector, It would be acceptable, but an aftermarket sound system solves that issue. I believe it is a candidate for the smoothest motor I've ever owned, including two GL1800 Goldwings, and a six cylinder CBX. The GT(L) is very agile for such a big bike (parked next to a sport bike, it looks HUGE), and a joy to ride on ANY rode, although as usual, secondary roads are more fun that the interstate. Fuel mile is pretty similar to the GS(A) during local riding, but at Interstate speeds (75-80 MPH) the slippery shape of the GT(L) allows it to consume a bit less fuel that the GS(A). Passenger mounting is pretty much routine on the GT(L), and the massive torque this machine has makes getting the extra weight up to speed effortless. (0-100 is one short scream LOL)
Bottom line: with a few mod's, either makes a fine tourer. Stock, I'd give the nod to the GT(L). I'd rather ride either one than a Honda Goldwing, although there's lots of riders that would disagree with me. I'd suggest that you try a four hour plus ride on each, then decide. A SHORT ride on either won't tell you much, especially on the GT(L), whose electronic throttle and plethora of gadget take some getting used to.
Hope that helped.