All the manual machines are very similar, the main difference lies in the width and thickness of dies they can take - and this difference really all boils down to whether it will cut Sizzix Bigz dies or the biggest of the Grand Nesties. Other than that they're pretty much the same.
Big Shot and Big Kick are exactly the same machine, and the Vagabond is the same but runs on electricity. I don't think the electric Big Shot is available any longer. They will take everything on the market except the largest of the Grand Nesties (which is really only a couple of the very biggest dies).
Cuttlebug is essentially the same, and I believe the Epic/Revolution is as well.
The Sizzix Sidekick is a petite version and will only take narrower dies.
Grand Calibur will take everything except thick dies like Sizzix Bigz (which includes many Tim Holtz dies).
Big Shot Pro will take everything.
Fiskars Fuse will apparently take everything (it's coming out in a month or so so it's still an unknown, but it's priced like the BSPro).