I only have EZDrummer, so take this with a grain of salt.
EZ Drummer is basically a dumbed down version with slightly lower sound quality. The drum samples are largely pre-EQ'd for mixing, although you can adjust some basic settings, such as room size, levels on a mixer, etc, and you have some limited ability to switch drums within a set (most have alternate snare, tom and cymbal samples for example).
It also comes with a lot of beat packs... generally 8 or 16 bar beats that can be cut and paste into DAW to build a drum score.
Superior Drummer (to the best of my knowledge) lets you get right into the details, so you can treat each sample like a separate input on a recording console and edit accordingly. You can also record your own or download other people's samples and mix and match freely if I remember correctly. I'm sure there's a lot more to it and hopefully someone else will chime in.
I went for EZDrummer due to the simplicity. I like it and it serves my purpose well.