The general rule is to use a colon when you're filling in fairly mundane details to clarify or extend what came before, whereas the dash is more dramatic, used to set off a surprising or important bit of additional information. (See http://www.quickanddirtytips.com/education/grammar/dashes-versus-colons?page=all and http://www.fas.harvard.edu/~wricntr/documents/GP.html). I think this is pretty clearly the former case, and the dash reads badly to me - but it's your bikeshed, paint it as you like! :-)