differences between free and paid virtualmin?

All you need is here: https://www.virtualmin.com/vs

Diabolico's link is a good one. But, it's also missing a bunch of stuff (I need to update that page). But the big differences are:

1. Install Scripts: 100+ vs 7 in GPL
2. Reseller accounts and additional ACL flexibility provided by a pseudo-admin account that can manage many domain accounts
3. Additional user limits for stuff like bandwidth, CPU/memory, etc.
4. More stats.
5. A bunch of little niceties all through the product that Jamie never told me about, but that I find every once in a while when I tell someone to use some feature to do something and they say, "I don't see that feature" (because they have GPL and I was looking at Pro when I offered advice).

We're always merging stuff down into GPL, but there's a core set of stuff that's focused on people who run businesses on Virtualmin that will probably always be in Pro only. If you're making money with Virtualmin, or hosting untrusted users, the Pro version has a lot of things that are useful for you. Very large deployments are probably also well-served by Pro (again because of the reseller account type, which allows a bunch of delegation options that GPL doesn't have).

Edit: Oh, and of course, support. Pro customers can file support tickets in our tracker, and expect to usually get an answer the same day. We try to help people in the forums (and in various other places), too, but we're pretty much always overwhelmed with just supporting paying customers, so we have to prioritize the issue tracker.