[silentbicycle]

An addendum, a couple months later:

Another major difference is that Emacs frequently uses modifier keys, and it is especially common to type with Control held down (while navigating). If you use Emacs, you will probably benefit from moving Control to either CapsLock (home row, pinky) or the left Alt (to hit it with your thumb); bending your pinky down to Control at the bottom of the keyboard while typing contorts your hand rather badly.

On the other hand, Vi seldom uses modifier keys besides Escape (which could also be moved to CapsLock), and it is never held down while typing. While I don't know of any hard data, the default Emacs configuration is almost certainly harder on your hands in an RSI sense.

Wily (a clone of plan9's Acme: http://www.cse.yorku.ca/~oz/wily/index.html) is another extremely powerful editor, though I don't personally like its mouse-centric design. Still, it's quite extensible, and probably worth a look.