Looking at the uses for each tag, most of them seem to be about the same use above. The use of each tag depends on which wording the author of the question used (was thinking of at the time they wrote the question). The only slight difference I can see is some questions using the registration tag asking not about how to sign up for the site, but about converting an unregistered account into a registered account, but the process is still the same (you do so by signing up for the site using email/password or a login provider, as stated in the sign-up tag's description).
Should these tags be merged? If so, which tag should remain (I think sign-up because it has more questions and a tag description)?