A VERY interesting question ! Thanks for asking such thing :)
Both methods are polyalphabetic ciphers. There are some algorithms for polyalphabetic ciphers which computes key size from ciphertext. Once key size is known, ciphertext can be splitted into columns which are vulnerable to simple frequency analysis.
So answer is that both methods have the same level of security. http://www.security-forums.com/viewtopic.php?t=15961