How important is the choice of dissertation committee?
I can share my perspective as person who recently completely a PhD thesis (May 2015). I think that choice of members is something which should not be taken lightly. I would think about a couple of key components. Which members of the faculty have interests/experiences in areas which are potentially interested in pursuing the future? They could be useful in terms of networking in the future.  Are their members of the faculty who you have spoken to about your project in the past who might be able to bring a bit of familiarity to defense, this is potentially important in terms of them being able to write a stronger recommendation letter. I think the last thing to think about are the different components of your thesis and selecting members who can best ask you about the individual sections in your thesis. 

Best!