There are a number of differences between the "free" and "paid" versions.
Free Version
-water Effects
-Water Bubbles (1 variation)
-Two Timer system (When we get it working right)
-Better performance
-All script commands
-noncommercial Games only
-Can be used in contests
-Splash Screen
-Watermark in built, and test games
Paid Version
-water Effects
-Two Timer system
-Water Bubbles (multiple bubble image choices)
-New blood splatters
-New free sound effects for included scripts
-Loads of Pre-made scripts
-Lens Flare
-Reworked Ally System
-Commercial games
-Can be used in contests
-No splash screen
-No water mark.
Thats just some of the things.
We will have an official list made later.