None. They're the same thing. Whether you choose to spell it Bahai, Baha'i, or Bah?'?, it's still the same religion: the Bah?'? Faith.
http://www.bahai.org/
It is the same thing. The correct spelling should be Bah?'? but the apostrophe and the accents are often forgotten for reason of speed or for compatibility with old computer systems. Some keyboards for example only provided unaccented characters.