That's a very binary way of looking at it. Depending on the sign up flow, it may be much more difficult to discover the existence of a username or not, than it is in the login form. (For instance, if the signup flow involves payment details before the username step.)
Are you going to enforce uniqueness of payment details? And if so, what part of those details must be unique? Will you require a unique credit card number?
If so, you will have to maintain either a bloom filter or a list of hashes for all eternity to ensure that nobody signs up twice with the same CC number. [Edit: actually, you could keep the scrambled CC data for a finite length of time if you're just trying to rate-limit, but still see below]
So suppose you've done all that. Are you going to do a test charge to see if the credit card number is valid before allowing the user to choose a username? If you don't, your efforts are wasted, since anyone can just use a CC generator (they're used for testing) to sign up for new accounts all day. If you do, you've probably driven away 99% of your users with your ridiculous signup process.
If you're using a third-party payment provider like Authorize.net or Braintree to store CC numbers, you'll be given a unique token from the provider when you add the CC number. When you add the CC you can also have the provider run and immediately void a dummy transaction to verify the card is real, too.
