which dart is more disirable
It all depends on what your future plans for the Dart are. If you're going to keep it all original I think the GT is the best choice. But if you're planning on building the car to be what you think it should be, then go for the plain Jane.
I wouldn't want to mod a survivor Swinger 340, GT, or GTS, so I found a '69 Dart 270 for my build. The previous owner had already started a restomod build but had tired of it, so I bought it and will build it to be exactly what I want.
Choose your direction, then plan accordingly.