How do you choose? Well I would suggest you go on pinterest or Houzz and find pictures you like - really look at them and see what it is about them that you like.
I have an iridescent blue glass tile on my spa, and it iridescence adds a lot of variation to the tile, you'll see burgundy, pink, greens reflecting depending on where you stand and the time of day. I found pictures don't tend to do an iridescent finish justice, which makes it hard to choose if you need the visual.
There is no issue doing stone with a salt pool as long as you know that you have to seal it every couple of years and are diligent about doing so.
Texas pools seem to be popular to use flagstone coping too, possibly because its an affordable material choice in that area.
The picture you posted I'm guessing for inspiration is very monotone. I went for a monotone look on my tile. How I chose mine was that I picked something first - in my case the coping because it was natural, and was subject variation, so I picked out a pallet at the stone store. Knew it needed to be sealed, so took a sealed sample to the tile store and matched it in color and character. Then I went around the tile stores with the sample and did the same thing for my wall tile around the spa. I also knew I wanted a pop of color, so to pick out my glass tile, I chose that against my already chosen samples.
There is no hard and fast rule, some people fall in love with a certain waterline tile and then match the coping to best compliment that tile. But if you have literally no idea where to start I think perhaps the best place to start might be the coping, its probably the most noticeable part of the pool from a distance and creates the most impact on the style of the pool because it represents the transition from your pool to your deck. In the picture above there is a travertine paver deck, and the look is virtually seamless which means choosing a travertine coping stone. So your choice of deck material will also carry into the decision.
Editing to add, if you want to see what a wall of blue iridescent tile looks like find cchopkins' long and skinny pool build thread. Or mine. Same tile, NPT Arctic Lagoon, cc's pool makes it look fantastic though, better than in mine.