Benefactor = First bounty you manually awarded on your own question.
Promoter = First bounty you offered on your own question.
What is the difference? Sometimes I think that the explanation of the badges are not so clear...
Promoter = You started a bounty
You can start a bounty but let the system auto assign it, so in that case you'd get only the Promoter one.
Instead, if you manually award it, you'd get both.
EDIT: As Tobias pointed out, you can get the Benefactor for a second question if you didn't award it in the first.