I have done the ST and the TA so I can offer an opinion based on first hand experience. Bear in mind that it is just one opinion though.

I found the scenery mostly pretty dreary much of the way on the ST. I like trees and other greenery. I also like lakes, rivers, and streams. The ST had very little of any of those for more than half of the route. It was somewhat made up for by great food, nice people, and the ability to do the ride in winter. There were very few places that were pretty west of Del Rio IMO. It got better east of Del Rio for me when things greened up a bit and I liked the Gulf coast pretty well.

The bottom line for me is that I would never do the ST again unless I really wanted a winter tour. The TA on the other hand I would love to do again. I can't speak to the NT from personal experience, but suspect it is more like the TA.

Edit: I should have mentioned that I do know folks who love the parts that I consider completely bleak and actually prefer them over the places I like.