There are 4 flavors of muff from EHX, there is a 5th one coming out later this year that sounds cool, and Dunlop/MXR is putting out a fancy one under the brand name Way Huge called the Swollen pickle. But those 5th and 6th options probably won't be available at GC immediately.
NYC Big Muff
Black Russian Big Muff
The Little Big Muff
The Bass Big Muff
I have a little big muff. Really fat and mean sounding. I've played the black and the NYC muffs- they're both a little bit tamer, the black muff is aggressive sounding but not as full, and the NYC muff is full but not aggressive sounding. Little Big Muff is sort of inbetween.
Bass Big Muff is based on the old green russian big muff that bassists really seem to like (and guitarists, I believe thurston uses a green muff), but with an added bass boost and wet/dry blend mode (so that you have more definition.) I keep hearing that this one is really awesome and better than the little big muff. I'm going to get one myself.
One last thing about the little big muff and bass big muff- they're smaller, and they have normal power connectors, which is nice and convenient.
The Way Huge Swollen Pickle and Big Muff Pi with Tone Wicker are going to be really cool. Both of them have extra features to give the Big Muff tone more flexibility. I want to get both. I'm a muffaholic.