IMAP supports push email via Push-IMAP, so it's incorrect to say that "push is faster than IMAP". You can set a POP client to check email every minute which will remove any delay perception for most people (it will also increase your data flow marginally, but we're looking at unlimited data plans with iPhone anyway).
The bigger advantage of IMAP for me is that all your folders get synced to the server, so you can see your folders, Sent items, etc. wherever you check your IMAP email form.