I'm a big Direwolf20 fan (although I like Etho's videos as well) so I chose the direwolf20 pack to be able to rebuild his contraptions in my world. From reading the forums I got the impression that Mindcrack forces you down the path set by Gregorius, while Direwolf20 gives you the freedom to build what you want earlier on. I also like my Mystcraft mining age. I'd hate to have dozens of huge holes in my overworld. So I'm happy with my choice. The only thing I'd like from Mindcrack is the fusion reactor.