Yeah, this question has been coming up a lot lately . . .
The basic motor, chassis, and electronics are the same on the GT & GTL. It's mostly the riding position, plus the trunk and a few accessories that differentiates the GTL from the GT.
The GT ergos are closer to the RT, with shorter handlebars, a taller two-piece seat, pegs higher and further back, and a shorter windshield. It's leaned more toward solo sporty rides, and the lack of a trunk makes it a bit lighter.
The GTL has longer bars, a one-piece seat that's lower for the rider and wider for the passenger, lower/forward pegs, and a taller windshield, plus the trunk which makes it a bit better for two-up rides. The GTL also adds LED driving lights and some chrome trim.
With an experienced rider, either model can be ridden very quickly on almost any road, either solo or two-up.
There are basically four K1600 models:
"Standard" GT model - as described above.
GT Sport - lower windshield, custom paint, and a different seat.
GTL - as described above (adds trunk and driving lights)
GTL Exclusive - GTL with custom paint, and all the options: more lighting, luggage liners, and some nice two-up touches like armrests, and a wider passenger seat.
The biggest obvious difference is the riding position. It's best to sit on (or test ride) both, but if the GT feels a bit too leaned-forward, then the GTL might work for you. Alternatively, if the GT riding position feels good, and you're less interested in the trunk, them the GT is a good option.
The trunk alone retails for $1,585, so if you think that you'll use that, it makes the GTL a good deal. If you primarily ride two-up, then the Exclusive is worth a look.