How different are the two (if you've used the Ubuntu version as well, that is)?

I've got my family running Linux Mint, and they find it suits their purposes (as any well-designed mainstream Linux distro these days should!). I've been wondering if it's worth switching them over to the Debian-based version the next time I upgrade it, though.

(Linux Mint encourages the "Windows-style" upgrade - ie, back up your data, then install over, and restore, so it wouldn't be that much more work than upgrading to the latest version of the Ubuntu branch).

I agree completely. I've been running LMDE for a little over a year now; quite easily my favorite distro.