[softwareman]

We have already had a hard time selecting a email username (the part which comes before @). Now if you ask me to choose another username, I am going to be wanting the same email username because I do not want to remember another name. Its more difficult than remembering passwords. If your service becomes extremely popular then I will not get a proper username available (if I am one of the late to party guys).

So avoid usernames at all cost. Email is longer, but more often, who needs to enter it again and again, we anyways store it in stored passwords on chrome etc.