Hi PTheory,
I actually own both along with several expansions for each. I also own NI's multi-out kits and a few other titles. From a purely functional standpoint I think BFD and Superior are very similar and both have good, usable sounds. In my view the differences are largely a matter of taste. I find that I'll go with SD over BFD most of the time, but again, that's a matter of taste.
Some functional differences:
BFD ships with a larger library of drums (IIRC), so you have more choices out of the box.
SD has a larger mic selection. Most of the SD libs have the usual mics, but also multiple room/ambient mics as well as special effect mics like trash mics or mic's that were run through classic compressers. These mic choices are mostly helpful if you're getting really into super-unique artistic creation with your drum sounds. If, like me, you're going more for usable acoustic kits in a reasonably short time frame these will see less use.
SD has a Multihit control that changes the way drum hits that occur during the ring period of of a previous hit on the same drum occur. That is, if I hit a cymbal, and while it's still ringing, hit it again this control changes the attack on the second hit. I've found this VERY useful in cymbal rolls.
Both BFD and SD have minor UI differences that I prefer. Sometimes I find myself saying "damn, I wish SD did <x> the say BFD does" and sometimes it's just the opposite.
These practical differences are really minor though. It's an aesthetic preference for me. I just like the sounds better in SD. Others like BFD. For me the BFD snare drums are almost all too tonal and ringy and the cymbals are somehow different. Some folks will disagree with me....and that's really OK, they like different stuff than me. Best bet: listen to both.
Also, as long as your shopping for good multi-mic acoustic drum libs, you should really have a look at NI's Abbey Road and Studio drummer kits. I like those a ton. And be sure to check out our own Jim Roseberry's Jet City kit if you own NI's Kontakt. It has a sound that none of the others do and he handles hit position on the snare drum in a way that no one else does and everyone else should steal. It's a super cool product with a great price point.
Good luck
Dean