Emacs is a better IDE than vim, but vim is a better editor than emacs.
Emacs is far easier to script and plays nicer with other programs so there are more, more useful extensions for working in various languages. On the other hand, I find vim's modal interface faster and easier on my hands than emac's chording interface. viper-mode in emacs might be ideal.
as a textmate to vim to emacs user, I'd say that vim is to textmate as emacs is to vim. Vim is awesome, but it's still just an editor. Emacs is an operating system that's built around editing text. You start with a powerful editing paradigm and then apply it to pretty much anything you need to do as a programmer...it's about the best editing experience I've ever had. My shell, source control, organiser, messaging, documentation, directory browsing, irc, document viewing etc is all done from within my text editor with all the awesome editing capability that 23 versions brings with it...also you can run vim from within emacs...
Exactly. Emacs didn't start on Unix IIRC and seems to prefer to have everything built in. vi was created by Bill Joy on Unix and wanted to work with the rich language of shell commands from the start, as ed(1) did, so it concentrated on editing text well and integrating with Unix well. To see some Emacs users you wonder why they're even bothering to run on Unix. ;-)
Ergonomics. There's an emacs key chord for everything, but many of them involve two or three modifier keys.
i.e. search and replace is Alt+Shift+5, indent block is Ctrl+Alt+\
These require some finger contortions. I haven't used vim, but the impression I get is that similar commands in vim are all closer to home row so you don't have to contort your hands as much.
I use emacs for most of my text editing. I hate using vim because I find all the mode switching to be really annoying. I like how keys always mean one thing in emacs, even if it results in the occasional bit of emacs pinky.