I assume you are referring to the Android version of OpenCPN.
The paid version is from the lead developer or the team, not sure which. He and his team have put in a very large amount of time over more than a decade to make all versions of OCPN the great application it is today. The $10 is a pretty minimal expense compared to the effort.
The unpaid version was offered by someone who apparently wanted to make the point that FOSS should be available free. That is a valid point. I think his intentions were good but it didn't go down well with some in the community. This is now old news but if you really care then do a search.
I think it is reasonable to assume that the paid version is more current
, but that is easy to check if you do just a little digging. FWIW I think paying a little to the devs is the appropriate thing to do. YMMV