If it was me doing this (and I wish it was but I don't have the skills) I would go for the smaller board. Reason - the difference between an 80ltr and a 104ltr board isn't going to be that big (24 ltrs to be exact), which will reduce further when you go down in size like you say you will do soon, so the difference when that happens will potentially be negligible. The difference between the smaller board (whether 45 or 65ltrs) and whatever size 'big' board you end up with will not only be noticeable, but also will give you a totally different sensation when flying, AND will give you a chance to make the most of different conditions - a light wind board and a board for howling conditions maybe! I don't see the point in having two boards that are so similar in size, even if they are different designs. I know your 'big' board will be SUP specific rather than wing specific, but IMO a good SUP board can wing well, but a wing board isn't necessarily good at SUPing.