Given that you're inclined for a "color" Knot, I'd select based on the total effect of the combination of size, color, and material. For me, there are certain combinations that just look prettier than others, for the particular purpose in mind.
For example, I have a Stretch Knot in nero (black) karung, which is softly reflective and textured, so it goes day-to-evening. I also used to have a satin regular-size Knot in a champagne color, which was perfect for an outfit that I wore for a family wedding. I would not have wanted the faille texture for that outfit in that color, but the satin looked perfect. It was so elegant. The satin is so smooth that I babied it to avoid fuzzing or snags from dry skin, for instance, but that was partially due to the fact that little flaws would be more noticeable on the mid-light color. I wouldn't have worried that much over a deeper bright color.
Size-wise, I like the look of both, and like I said, certain colors to me look better in the smaller shape and others look better (to me) in the elongated shape. To be completely practical, though, I'd go for the Stretch size if you need to carry the iphone 6s, especially if you plan to use a card case, too, plus carry essential keys.
Have fun deciding!
