The first one only hints at the question possibly being suitable for Super User. At most it tells the OP to look into that site and see if it's a better fit for the question asked.
The second one however is a direct vote to migrate. If all votes are of the migration kind, the question will end up being migrated to the target site. This won't happen for the first option.
Why have both? If you're not 100% sure that a question is a perfect fit for a target site, you should not vote for migration. I often have an idea it might be a potential fit for a site, but I might not be certain. In that case it's better not to explicitly vote to migrate.
Additionally, a question's subject matter might be a better fit for a specific site, but it might be a poor question. Voting to migrate crap is also something to avoid, so once again, the first option would be a better one.