I’ve been using Tailscale because I like the business model they’ve been pushing. They’ve also followed through with that, expanding what the free tier of their service offers instead of restricting it more.
Although possibly biased, Tailscale has a write up comparing the two and the conclusion is that both are good at accomplishing their goals. If you have a technology or business preference, it could mostly come down to that. tailscale.com/compare/zerotier/
As modern enterprises migrate to cloud-based services and remote work, fortified corporate walls are deteriorating, and so is the use for a traditional VPN. Network architects are embracing a new “zero trust” approach, which means physical networks cannot be trusted, and every device must always be end-to-end encrypted and authenticated.