i like them both. black widow is rarer definitely, you'll see more robotrons before you see another one of those. if it's a 'which do i get' thing, i'd get the black widow and keep an eye out for robotron.
Both are difficult in game play. BW is harder IMO. Did they even make a BW cocktail? I've only seen an upright. In the end I'd go with Robotron. But if you can get any BW cab for cheap buy it, I doubt you'd have a problem flipping it.
I'd vote black widow.. I've only seen a 1 or 2 up for sale here since I started
collecting. I've seen 3 or 4 robo minis up for grabs over the past year through out the country.
I think they are equal in game play.. black widow ramps up slower but gets
just as crazy at higher levels.