That does not nearly have the same security level. Signing a tag in git is like signing a commit. A commit is nothing else than a reference to a tree SHA1 (and parent, message and author). That tree SHA1 contains SHA1 of the files and trees in the directory.

Even assuming your client checks all the SHA1 and does not just trust the server (which is not the default), and that you are verifying the git tag signature correctly (which is harder than just checking a tarball), SHA1 collisions are considered practical.