IMAP is clearly the way to go. Here's a good, short explanation of it. Basically, IMAP means that your email is 'out there' somewhere and not subject to the storage limits of your local device (PC or 770, for instance) and you can access it from any device no matter where you are, as long as you have an Internet connection. If you've ever lost your email because your PC's hard drive failed, or because you bought a new PC and you didn't know how to move your email to your new PC then you suffered from the POP way of doing email and you would not have lost your mail if it had been stored and manipulated on an email server that was remote to you, and available to you.

When using IMAP you can avail yourself of all the spam and junk mail filtering on the remote system. A single person could easily set up an IMAP email service that would contain all the protections that we need in email, but instead of us all implementing these protections individually on our 770's, these are implemented just once, up there on the IMAP server.

Although Google doesn't actually use IMAP it does offer a version of POP that imitates the way IMAP works. Here's a comment from Reggie regarding 770 email.