Hi Mark,
Both boards will work ok for Winging - the longer 6'9" will always be a bit easier at first as it is more stable and especially for SUP foiling it will feel better. But later for winging the extra length will not feel as good as the shorter 6'3". Just for straight paddling with no foil there are better boards for sure, if you want an allrounder that also does that - then look at the Bee - SUP/SUP Foil/Wing/Windfoil.
Construction wise, no big difference between the 6'9" and 6'3"
Prone or SUP foil on the 6'3" - possible but it is made for Winging, so the shapes are slightly different/it is shorter making it harder.