The paid version has three times the content, an overhauled story, many new levels, an extended soundtrack, etc. (basically more of everything)
The free version was done for a competition, so was done during a short amount of time.
Viewing post in Dango Dash (Game Jam Edition) comments
And how can a digital (!) edition be "sold out"? Because that's what it says currently. I don't get it, why is this happening to so many games? Same with Shapeshifter 1 & 2. This way a few buyers enjoy them and then the game disappears from planet earth forever, which is a shame. I would have gladly purchased the digital version as well.
Then I apologise, although it's pretty bad from a UX perspective to write "SOLD OUT" (instead of something like "Pre-Order") underneath the product imho but I guess this isn't your fault. :) Looking forward to your release.
Edit: Well, apparently they changed it now. Also I subscribed to the mail notification to get an alert when it's out. :)