ST has about 20-30 more miles in range, (smaller tank on the GT possibly less mpg), non ABS available, around 10lbs lighter, with a 2 inch shorter wheelbase for snappier handling one up, with slightly higher pegs and seat for more lean angle.
So for us who never ride two up the ST does have its positive attributes vis a vis the new GT.
I'm guessing the GT is better at two up riding and it sounds like the ABS has been improved.
One thing I never thought the ST really needed is the Traction Control available on the GT...but maybe that massive 5hp increase in power requires it.
The ESA option, according to owners of other BMWs with it, is not the "home run gotta have it" feature that it sounds like it should be.