Hello Vhale,
the only difference is whether or not you are legally allowed to use it commercialy. So if you're not sure pixelator is right for you, you can download the free version, play with it a little, and if you see it's good for your needs - buy it commercialy. It will have the same features.
Ofc for non commercial projects or if you're under 18, no need to buy at all.
Hope this clears things up