I use a gig bag because it's so light to carry. But if I'm going to play a gig somewhere far from my home and I must go there by buss or car which has a lot of passengers and/or a lot of heavy stuff I prefer a case because it gives a good shelter to the guitar.. If you are moving I suppose it could be wise to keep the guitar in the case instead of moving it around, because a gig bag doesn't offer so much protection..

A very intelligent answer.. Duh. :)