No idea why the salesperson said that. I don't think there would be much difference. I think either one would be good.
Two other differences come to mind: One is the lower front cargo carry on the Tern since the front wheel is also 20". Not a huge deal, but if you're frequently loading up the front rack, this might be noticeable. Second is the ability to easily transport and store the Tern. It's shorter overall, can fold down to a smaller package for loading into car or other places, and can stand vertically. If these things aren't things you'd take advantage of, then they don't really matter.
Also, the GSD is also offered with the CX motor on the S00 model, which also comes with the Nuvinci N380.