During a registration process, what would the reasons be for using either username/password or email/password combinations to register the user?
What other reasons are there for choosing either option? Is there a relationship between the types of service?
For the same number of fields (2) you get more useful information about the user. Both username and password are content free, but an email address is not.
In addition, my experience has been that using email address rather than a username bypasses people's privacy filter. Many people avoid giving out their email address, or use a throwaway address when they are asked specifically for it in a form. But when it is their identity, they are more likely to use the address that they think of as 'them', which means not a throwaway.
Finally, in forum/comment environments you also need to ask for a Display Name; phrased in this way, people are more likely to use their real name than an alias. It serves the same purpose as the original username, but again in my experience a far larger percentage of users provide their real name, but since they don't have to provide their real name it does not turn away or offend those who value privacy like 'First Name', 'Last Name' fields do.
If the service has a social/community aspect, then allow creating a username. (This doesn't have to be during registration. You could give them a default until they want to change it.)
If no social, soley an email allows them one less username to remember.
Finally, the login field should allow Username OR Email.
Quote from this article on ask-leo.com: "Your username and password identify you to your email service, and your email address identifies you to the world."
If your application has anything related to sending the email then it is better to sign the user with an email option. If you want to refer to the person only, without having him to contact then you need to simply log him with username. Also, it would increase security for a user.
Logging in with an email address plus password is likely to be easier for the user most of the time (but see below). People might not want to broadcast their email addresses, though, so if this is a site where user names are displayed (ratings, seller/buyer names, social, etc), you'll also need to provide for a display name. Note that Amazon, eBay, and Google work this way.
One caveat, though: email addresses are not necessarily permanent, and you don't want to force someone to create a new account if it changes. If my address is [email protected], that address isn't going anywhere and I'll use it everywhere I can; if, on the other hand, it's [email protected] (or [email protected]), that's a weak foundation on which to build an online identity.
Therefore, my suggestion is to have users create any user name they like, including using their email address for that purpose if they choose. Failing that, my second recommendation would be to use email address (plus display name) and provide an account-migration feature if the email address changes.
For me personally it's about giving my complete details before I'm even a member. I'd rather just sign up with email/pw and then later figure out how I will fill out my profile, as me or a more obscurely titled me. Example: for Facebook I use my full name but on here I've abbreviated my last name.
I think opting for email/pw will give you a higher influx of sign-ups because it requires less thinking. 90% of all apps/startups/sites I apply for are email/pw sign-ups, or social (fb/twitter) sign-ups.
But that's just personal and not backed up by any study or anything...
One immediate advantage of specifying a user name in addition to the email address is that the email address won't be what identifies the user in the application and can remain hidden and not accessible to other users of the service. This is an integrity issue and there are many potential users who would not sign up to a service where their email address would be public.
So for registration the user should specify both email, user name and password. And for signing in they should specify email and password. This is because there could easily become a discrepancy of user names the user have for different sites, simply because the intended user name is taken or that the user want a "new" user name. The email address however is more stable in that regard and most users won't have to ponder "which email do I use now again..." when they sign in.
@ in the username, but prevent people signing up with other values than dash, underline, A-Z, a-z, 0-9.