Compared to the free version, the paid version currently has these extras:

• Export to PSD and SVG format instead of just PNG.
• Batch export (if your character has multiple hairstyles, expressions, or clothing styles, you can export all of the different combinations to a folder).
• Multiple layer export settings. For example, with 'Face Only' you can export only the face parts, with 'Clothing Only' you can export only the clothing parts. Afterwards, you can combine those separated images via the game engine (for example, layeredImage in Ren'Py).
• The full version is able to use downloadable contents (either made by us or by other people). This includes Steam Workshop for the Steam version. Purchasing in itch also gives you a Steam key which can be obtained by following these steps.
• All downloadable contents made by us are included with the full version (in this early access phase, all DLCs are already present inside the main installation, but in the future we will split it into separate downloads for better file management and update process). Currently, these DLCs include Christmas Collection, Long Hair Collection,  Animal Ears Collection and Summer Collection.
• Developer Tool to help you create your own templates (still in progress).

The 'nightly' version is more frequently updated with in-progress features (almost daily), but has the risk of more bugs. This is for people who want to be more involved with the development progress.

Hope this helps!