Username or Email

[eluser]Michael Wales[/eluser]
If the identifier (whether it be username or email) will be displayed publicly, I always choose username - to combat spam.

If it will not be displayed publicly I always choose email. Every user knows their email address - usernames... I have about 10-15 of them and I can never remember which name I used on which service.

Of course, client's needs always override my choices (although I will present my opinions to see if they change their mind).