Hi,
We are going to explain all of this in the coming weeks, with a system that should make everybody happy we hope.
But let's say that when the new version comes, the old one won't be accessible anymore. So the upgrade cost from SP 1 to Live will be a bit longer, as it will be from SP 1 to SP2 and Live.
That said, I would suggest you to go with the monthly payment, as it will remain $19.90 per month. It's just the "payment duration" that would be a bit longer.
Cheers,
Nicolas