I went finning yesterday after my foiling session, 2nd time this year. The guy crib footstrap video motivated me to go for some reason. Anyways, it is definitely fun, the speed difference compared to foiling is noticeable,and getting air is cooler for sure on the fin.
I was a little surprised at how much more physically challenging it is to windsurf versus foil. I thought that easily being able to handle all these two hour foiling sessions, was because I was fitter....but actually foiling is just much less demanding.
I find both very close.
WS, usually in 15-30 knots, about 4 days a week.
Foil, about 7-18 knots, 1-2 days a week, needs constant tiny adjustments, which wear out my skinny, 71 year old legs with bad ankles.
My hand and arms are fresher after foiling, similar to SUP, but the legs and feet get worked, compared to 40 jump 1.5 hour WS sessions.
I know its completely contrary to what others find, but i actually find windfoiling takes it out of me more than normal gear.
i probably do a better job of tuning my normal gear, which means i move around less on it, that and i find theres a lot more body movement when foiling anyway, even with tuned up gear. I suppose it depends on how hard you're trying too.
Certainly i got thrashed the first few months of wind foiling. The great thing about windfoiling is the larger number of hours you can ride in foiling conditions, you get more sessions and you are foiling more of the time during each session, allowing you to keep tweaking your gear. Also due to not having to deal with chop foiling is very predictable so you can much more easily refine your technique. Swells were a challenge for me in the beginning but less so now.
Depends a lot on the conditions, I'd say. If it's marginal and requires frequent pumping, foiling can be more work. But maybe I just give up quicker with a fin 
. Nice flat water and decent wind, and I can definitely go longer on a windsurfer. Enough wind and some chop, or very gusty wind, and foiling can be a lot easier.
Finally foiled with Naish and 4.5 after 8 out of the past 10 days windsurfing with sub 5.5 sails and mostly 85 liter boards.
Too much in gusts..over 22, not enough thru lulls of well under 6. Hard work.
The other thing I noticed is pumping on the windsurfer with a 4.7 is a much more subtle thing than on my foil board with the same sail.
