Bessie What would be the ideal characteristics you'd look for in a "cactus cut dahlia"
When the back petals of a FD or ball cut dahlia begin to wilt it is not so obvious but when the back petals of a SC or cactus begin to wilt it is very obvious. So, even though the "actual" vase life of a cactus variety might be the same as a FD variety, the "perceived" vase life is longer on the FD variety. So, I would looks for a cactus seedling that holds its form longer. Alternatively, a cactus seedling that can be cut when immature and will continue to fully open.
I have had good luck selling Hollyhill Black Magic at the market and, even though she is not a straight cactus, she might make an interesting cross with HH Ambiance.