One selects a different package, same source repo.
The other completely changes the installation, invisibly to the user, potentially introducing vulnerabilities.
Such as what they did with Docker, which I found less than hilarious when I had to clean up after someone entirely because of this idiocy.
The differences seem quite clear.
Because the separate installation means you can actually end up with both an apt installed and a snap installed.
My comment about docker was a specific example of such a case, where vulnerabilities were introduced. It was actually a commonly used attack a few years ago to burn up other CPU and GPU to generate crypto.
Yes, canonical provides both. Guess what? They screwed up, and introduced several vulnerabilities, and you ended up with both a snap and apt installed docker.
The fact that they are both packaged by Canonical is both irrelevant and a perfect example of the problem.