Not a BFD user here, but after watching others use it, BFD seems a little quicker and less fussy to set up and tweak to a good sound.
I
am a longtime Superior Drummer user, going all the way back to Drumkit from Hell, its distant ancestor. Obviously, I'm very happy with it. However, I don't automatically recommend it to everyone.
SD3 is more configurable than anything else out there, which may or may not be a good thing depending on your style of usage (if you want to get up and running and sounding good fast with minimal effort, check out Steven Slate Drums).
SD3 has lots of expansions but they are more expensive than BFD's.
As noted above, SD3 is HUGE, by far the biggest sample library I have. I do not run it from an SSD, and it can take quite awhile to load.
SD3 has features you may never use. It has its own sequencer, but I've never used it. It has a great drum-replacement feature that I've used only a handful of times. It has gobs of effects, many of which I've never used.
Bottom line is I'd say SD3 is for people who are really serious about their sequenced drums, who take their time tweaking sounds, and want no limitations. Although I fall into that category, I realize that not everyone does. Some just want to build a serviceable drum track quickly and then move on to the fun stuff, whether that's playing your guitar or singing or programming synthesizers.