Depends on the product you want. Amethyst is a standard snap bean comparable to Royal Burgundy or Purple Queen.
Velour is a fillet bean, which means an extra slim pod smaller pod. If you want skinny beans, Velour will fit your needs.Like the regular size purple beans, they become green beans when cooked.

I'm not especially picky about snap vs filet. I'm looking more for taste and the more stringless the better.

There is a difference. The fillet beans must be picked young 1/8 to 1/4 inch in diameter. That is smaller than a common pencil. They do have a much shorter harvest window. They do get tough quickly. Both types (bush) are stringless when harvested at the appropriate stage.