The programable version is like having three units in one. Its a great deal and sounds exactly like the single version.
Some prefer the paradriver as is has a mid control but the overwhelming majority seem to be quite happy with the regular version.
other than that, everything you could possibly want to know about the Sans Amp products is dicussed to death in these forums.
My advice would be to just buy one as they are an amazing tool and the smallest most portable back up rig you could ever have.
Some people use them as the hub of their tone, others use it as a standard stomp box for different overdrives and toneshaping, many use it as a standalone pre amp into a power amp or straight into the house mixer or recording console, and others just keep one in their gig bag in the event that their main rig fails, or is not available to them.
The Sans Amp will make a less then stellar amp sound better but many agree that it cctually degrade the tone of a very good amp. This is all of course subjective.
Hope that gets you started.