I prefer emacs to vi, but I'm comfortable in both.
There are some things that you can do in emacs that make it more powerful than vi, but not all of them are even programming-related. (Can you send email or read news from within vi? No, but who cares?) If you're comfortable with lisp (I'm not), you might be able to write add-ons and modes and stuff to make your life easier, but that's just likely to be syntax colouring and brace matching and eye candy like that.
I will stop rambling now. Will your productivity increase using emacs? No.
Update: See my comment below. Since I posted this, I have come across ways that using emacs has made me more productive than using vi.
