Hey guys — if you had to pick one of these boards mainly for prone foiling, which would you choose? I’m a 73 kg beginner. I know a wing board isn’t ideal for prone, but my situation kind of requires it right now. I don’t have plans to wing at the moment, so which one do you think would give me better use for prone? I could resell later, but I’d rather avoid that if possible.
4’4" x 21.5" x 3.5" — 44L
or
4’6" x 22.5" x 4" — 55L
I’d go with 4’6 just for ease of catching waves. If you are planning on catching the whitewash to chip in, either will work. Bigger will be a little more stable for the pop-up which is the hardest thing at the beginning. Start on small waves! 
If you really don’t have any other option, I would go with the 4’6 to learn on. However, why are you only limited to those two specific boards?
Depending on your surf skill level, I would push most beginners to go longer (but similar volume). More board makes things much easier on your pop-up when learning - and a little length goes a long way to make paddling easier / getting into waves earlier.
55L it too much for me. I vote the smaller one. Are you good at shortboard surfing? How’s your flexibility?
To give you some insight, I’m 10kg more and I learned on a 65L. So that would match your 55L more or less. HOWEVER, I much prefer riding a 55L and I could have learned on that. I also wing that 55 at 83-85L, which would equate to you winging that 44. BUT, the 65 covers more days for winging. If I could ONLY have one, I’d go with the smaller one.