In subversion the 'svn checkout' and 'svn update' operations are the same after the initial invokation. The only difference is that one gets the URL from the command line and the other gets the URL from the working copy. You can avoid worrying about one or the other and *always* do a checkout operation. That will work fine if the is an existing working copy already. Always doing an 'svn checkout' avoids the problem nicely.
Note that if one is checking out from a tag (which is supposed to represent an immutable "directory"), one can use "svn export" instead and avoid creating all the unneeded metadata subdirectories.