I've tested both. Very similar. Both are GNOME remodelled to be more Windows-like. Mint has its own fork, Cinnamon, with some nice new features such as a richer file manager and default accessories with proper menu bars, not GNOME's annoying "client-side decorations"; Zorin just adapts GNOME with extensions, AFAICT.

Mint is free and has a large, active, helpful community.

Zorin is paid-for.

Honestly, I can't see any good reason to pay for it myself.

Zorin Lite isn't bad, though. One of the prettiest, best-customised Xfce deployments I've seen yet, and it's free.