I suppose to make an informed choice on here you'd have to take stock of what Zorin OS' priorities are, what Ubuntu adds to the Debian base, and what parts of Ubuntu/Debian it is that Zorin OS is choosing to keep, modify or discard.
- Lots of apps target Ubuntu specifically and not Debian when they are providing .deb packages - what is the experience of Debian users like when trying to install packages intended for Ubuntu? Are dependencies satisfied in these cases?
- Ubuntu has put in a fair amount of work into Active Directory integration in Windows environments - does Debian get this?
- Ubuntu's Ubiquity installer is much more user-friendly than Debian's - is it more of an effort for the Zorin team to incorporate Ubiquity themselves than use the one already existing in Ubuntu?
- Are Ubuntu's packages really that much older? Bearing in mind that a distro like Zorin would want to be basing on Debian Stable, not Testing or Sid; Just checking Repology and it shows a package like Evolution is still at version 3.38.3 in Debian 11 and not available in backports. It's newer than the version in Ubuntu 20.04 LTS' repos, but presumably when 22.04 LTS launches it'll be in front of Debian again, until the next Debian stable comes out. I'd call this a wash with regards to which has the latest packages, which one is in front will just depend on where each is at in their release cycle. Both are roughly on a bi-annual cadence too.
- Ubuntu gradually moving away from debs to snaps - Zorin does plenty of its own packaging, so this may be a moot point.
As for removing Snaps and/or Flatpak, I think that's just a decision down to the distro maintainer. For an OS like Zorin targeted at 'normies' who don't care about technical aspects of packaging and wanting to present a very large software library out of the box, I don't think it'd be in their interests to cut out either one. I think that offering either/both is a very good value proposition to most users who just want software that keeps up to date on a system that runs stably.