Salt is not really about entropy. What salts must achieve is uniqueness: as much as possible, you must try never to reuse a salt value (even if changing the password for the same user). GUID are nice for that. Randomness is a nice method to achieve that kind of uniqueness with overwhelming probability (it is "nice" because it does not need any kind of worldwide directory).
For uniqueness, GUID-encoded-as-bytes and GUID-encoded-as-string are completely equivalent since conversion between the two encodings works both ways: two GUID that are encoded as distinct sequences of bytes will also be encoded as distinct strings, and vice versa. It does not matter that one encoding has "more entropy per character" or any such notion, because entropy is irrelevant for salts. (Or, said otherwise, when entropy concentration matters for a salt, then it is not a salt, but something else.)
However, existing password hashing algorithms and file formats can be picky about salts. For instance, the article that specifies bcrypt(*) says that the salt has length exactly 128 bits, so, presumably, if you try to feed a bcrypt implementation with a 36-character strings, bad things may happen (e.g. the implementation only uses the first 16 characters, ignoring the rest). The uniqueness of GUID is ensured only when the complete GUID is used, in the encoding that you chose.
On a more practical note, since both encodings achieve the same uniqueness, you may as well prefer to use the shorter of the two, to save some space in your database. Be wary, though, of any poor handling of binary data (e.g. using bytes as if they were a string which ends at the first byte of value 0). Conversely, since most password hashing algorithms expect bytes for the salt, you must take care to specify an unambiguous encoding if you want to store salts as character strings.
(*) While the article specifies bcrypt, the bcrypt authors tend to consider that the real bcrypt specification is the reference implementation; in that sense, the real acceptable salt length for bcrypt is "whatever the reference implementation accepts to use".