Of course for most packages the answer is trivial: if it's some tool or library, deb is the only option, while for some proprietary software snap is the only option.
It seems Ubuntu is heavily pushing and promoting Snap packages, citing advantages in packaging, isolation, security, updates, etc. yet all applications that ship with Ubuntu 18.04 are installed as deb packages.
One example is Firefox, which is a package that should be kept up-to-date for security reasons, and that in the past had conflicts with distribution package maintainers about security and branding (Iceweasel). Yet Canonical chose to ship Firefox as a deb package.
For packages that have both a snap and a deb, this raises the question: Which one should you install? And if the answer is snap, should I actively migrate installed deb's?