This topic has become interesting, as I've been challenging the same questions (with myself) as to the future.
I think the big variable in craft and wings for that matter that are ideal, comes down to conditions. Hawaii, The Gorge, Sth Africa and WA are always going to produce some of the best ocean conditions, they always have. As grom growing up it was always amazing to see guys windsurfing on tiny wave sails doing loops all over the place - meanwhile I am on a 6.5m pumping my arse off.
I have been downwinding in Maui on a SUP since it was a thing way back before boards came with deck grips. Even back then Downwinding was easy compared to doing it here in AUS (east coast) and I was doing them on 11'6" Nalu like prototypes. The first 12' Glide blew our minds in 2008, and it was progressed ever since. However in AUS, I do not DW on SUP at all. They don't even seem like the same sport to me at all.
Foil is kind of similar, except it is fun in AUS style DW conditions. Lots of fun. But what we are riding, much like how history has proved, is much different to what is regular conditions over there.
This said, I think the ultimate is Prone DW/Ocean surfing. And I have done some really fun runs, always with boat support/assist though. About a year ago, I rode with Ridge Lenny (Kai's brother) at Cronulla, we padded out into the ocean and I watched him catch multiple swells and ride endless waves to the horizon. I missed just about everything, leaving with one decent ride that I basically turned myself inside-out to get. Ridge said, even on an epic day on the Maliko run, you can be left sitting there for 20-30 minutes until you get the right bump get up on on some days. (normally a bad day on the Maliko is better than the best day in Aus).
Even on the best runs, you make mistakes and come off the foil. DW on a foil is very slow when you're "not on the foil", and very energy consuming.
For me where SUP has the edge (and I have done tonnes of both), is it is just so much easier to get more bumps, easier to connect more bumps, and I believe I travel faster and turn harder (for the same wing) than when I prone DW. plus I can limp home after the wind goes west on me which has happened quite a few times

.
The dream is still to prone from Manly to Palm Beach, and I will do it (hopefully) on the right day. But I think my SUP is the go-to weapon the other 99.9% of the time for now.
With your SUP Foilboard and Foil setup correctly (this is where a lot go wrong), your gear will feel light and perfectly balanced, fast and intuitive. The paddle is great for holding your GoPro so you can brag to your mates, and if needed can give you those tiny pops of extra power when needed.
This comment is neither either way. I am hooked on all Foil. But I am liking the convo, so thought I'd add my 2 cents worth. It is all still so new, and a lot of crew out there are still very green. I have done more DW runs than I can remember, but I do remember the one where I "GOT IT". Prior to this I had DW'ed plenty, and done relatively good at it. But when it really clicks, and I mean really clicks - OMG!
Anyway peeps, keep foiling.
Ride safe,
JB