Why not Karlsruhe? It has the KIT, one of the highest rated universities in the field of Computer Science, so lot's of people (hackers) there that might be interested.
(And Karlsruhe is about 64km away from Stuttgart.)
Few reasons, one of them: I know my way around here in terms of good locations for such an event, I think Stuttgart is very well reachable by train and there a quite some nerds around here, too. I don't see why not have another one in Karlsruhe later this year? Would be great to have another opportunity for leaving the office anyway :-)
And it's closer to the Max-Planck Institutes for Informatics and Software Systems.