In all reality, you're not going to see a huge difference in any of the units. The biggest differentiation among any of them is familiarity with the software.
XFI still uses C-Com, but it's a newer version of the software. It's also has the ability to log several other auxillary inputs, but you have to specify the ranges for each of them. I have dealt with one of the guys at F.A.S.T. on several occasions, and he's been so gracious as to give me his home number so I can ask him questions about a tune up. That, to me, is reason enough to use them. Not to mention that most anyone at a track will have some familiarity with FAST/C-Com where maybe not so much with the others.
Big Stuff 3 is supposed to be pretty slick. I've only seen it on one car yet. There is a guy in New Jersey that is running a twin turboed, solid axle Z06 to somewhere into the mid-8s with it. He raved about it and the support that he got from John Meany, but the product has already been snatched up by MSD so that may affect customer service. I'm not sure how many folks you'd find at the track running BS3, so you finding help from someone at the track might be difficult.
Gen7 DFI has one great thing going for it that I've always liked. If you change one of your sensors (O2, for example), you don't have to get a new calibration from Accel in order for it to work. You can calibrate the ECU from within the software. I'm not sure if this is something that has changed with XFI or not, but that's always been a hang-up of mine when comparing features. Gen7 also has a base tune up builder that is supposed to get the car to the running state so long as you can answer some basic questions about it. I've never used this feature, but would like to know if it really works. If so, that'd be cool from the standpoint that you'd not have to get a base tune from someone else and hope that it's close enough to get the car running.
I'm not sure if the XFI and DFI can read and correct off of two O2 sensors, but I think that BS3 will. Of course, that only matters if you're building twin setup, or wanting to read off of two headers.
It's interesting that I'd run across this post. Two weeks ago I was in Florida and was talking to an AEM rep about different systems, and he had a pretty interesting philosphy. He said that all of the systems are about equal in what they provide, and that there isn't one that is head and shoulder above another in terms of performance. He says to find one that you're comfortable with (software, hardware, etc.), and run with it. One of the things that I would look for is a plug and play option. I think it'd be much easier and cleaner to use a system that already has a prebuilt harness that will connect to whatever it is I'm building. Sometimes it doesn't quite work out that way, but in a lot of cases it will.
Sorry for the blathering. I'll crawl back under my rock now.