Steve, thanks for this very interesting topic!
I've been flirting with the SUP downwinding idea on and off for a year or two (strictly in my head though). If I were to try to put my thoughts in a structured way, I'd describe them along these lines:
1. Fun factor
2. Health benefits
3. Safety
4. Barriers to entry
5. Universality
1. Fun factor: it definitely looks very fun, at least here:
I think it may be even more fun than surfski downwinding as in car driving vs motorcycle riding (closer to the feeling of flying).
2. Health benefits: kayaking is a very symmetrical sport (left to right), but less so in upper to lower body. It's also hard on the lower back and legs for some (numbing). The SUP seems less symmetrical from left to right as paddlers seem to usually stand on one side. But it's definitely a much more full body workout than a ski. So all in all, again SUP is healthier.
3. Safety: I guess it's much easier to climb on a SUP and wait for help, if something is seriously wrong (laying on the board, paddling with hands)? Losing the board is more difficult due to the fin, the wind will carry away the ski very quickly, the foiling SUP will stay more or less planted. I think SUP is the winner here.
4. Barriers to entry: this is mostly the problem for SUP, as it requires a totally new skill set in terms of handling a board in waves. For an experienced surfski paddler reading waves is no problem, but learning to surf standing up may take some time. Of course, learning difficult new things is good for the brain (another health benefit), but it may upset many having to learn for months. Learning to paddle a stable surfski is much easier than to surf a "stable foil", if there is at all such a thing. Another aspect of this is living in a place with regular and sufficient conditions. Does foiling require super long swells? Are short interval waves tolerable? The ski wins this one.
5. Universality: this is probably the worst part of a foiling SUP - one has to plan everything perfectly to enjoy it. Go out with a fast ski and if the conditions are subpar to what was expected, the paddler can still enjoy the workout, having to put slightly more energy into paddling. With SUP it will go from everything to nothing. Maybe real pros and athletes can pump themselves out of the water in flat conditions, but probably the whole point of a foil will be lost for most. The ski here is the winner.
So, having said all that, it seems that a foiling SUP is more fun in the right conditions, healthier, safer than a ski, but needs a lot of learning and messing with guessing the right conditions. In that sense, is it more like kite surfing or windsurfing?