The GT is basically just an "improved" ST. Yes, it's a little longer and about 15-25 lbs heavier when considering luggage, but other than that, everything is better. The suspension is better, there's storage under the seat, the headlights are better, the bags are way better and larger, better highway gearing,, better for 2 up if you need it (no hard if you don't) etc. etc.
I would seriously consider an ST if you really like the undertail exhaust, want a color other than silver, blue or (sometimes) black, are looking to save some cash on an older model or have a big mental hangup about the wheelbase (see swartsie's post above). Otherwise, aside from cost, they are so similar with the nod going to the GT in my book. Don't get me wrong, I love the ST, but if an ST and GT were put in front of me, both for the same money and with the same miles, I would pick the GT every time, but wouldn't feel bad at all if the ST was forced upon me.
At the end of the day, you'd really need to ride both to get the full story. My advice with bikes is to always pick the one that "moves you" the most for the cash you want to spend. Sometimes that sexy undertail exhaust or blazing red color is what does that.