Hiya mate,
We've probably already discussed some of this but it might be a good idea to see if anybody else agrees/disagrees with it...
IMHO both have their pros and cons; the RIO will be by far the easier to learn the basics on due to the increased volume and daggerboard but you will out grow it quicker. Given that you are probably about 100kg(?) you would definitely want at least the biggest Go (177L?) and would benefit initially from the medium Rio (195L?); once you progress onto a smaller board you will only use a board of that volume in very light wind with a very big sail (or to teach somebody else), but the Go would be better due to no daggerboard slot and more advanced footstrap settings.
Unfortunately, I think it was said in a recent Boards or Windsurf magazine "the better a board is to learn on, the quicker you out grow it".
In an ideal world you would either buy a second hand modern board and resell it with little to no loss after a short period, or you'd at least be able to demo them both to see if what you have managed to learn on the Titanic is enough to allow you to manage on the more advanced board (i.e. Go).
Just a thought but I wonder what boards Onboard would suggest as they may have demo boards(don't think they do Starboard anymore?); also an hour or 2 of real instruction

on a modern board might get you to the point where you can manage on the go without the daggerboard.
If not, I'd probably go with the Rio as you are better off progressing quickly than struggling for longer than necessary, and you'll want a much smaller board than either of them in no time anyway.