IMAP is a newer protocol than POP, in that it allows for interaction with the servers, like you have with the browser-based gmail interface. POP just opens a connection with the server, downloads your mail, and closes down again, but IMAP can deliver new messages to your inbox as they arrive.
IMAP for sure. It is much more convenient to have the same set of email on every client as well as on the web.I switched over a year or so ago and have not looked back.
I prefer IMAP over POP, but have had weird issues with the Mail.app (os x), gmail, and my blackberry. Because Mail saves a draft every now and then, the draft gets sent to my blackberry, which can be bothersome at times.