No worries. Ask away. I love talking about RPGs. And Slay & Plunder happens to be one where I have a unique insight. ;)
Besides the art and layout, the paid version includes the monster list and treasure tables. It's otherwise the same.
The concept is that the Host (the game master in S&P) gets the paid version and the rest of the players get the unpaid version. But, if you are using monsters and treasure from other sources, the unpaid version is all you need.