Definitely decide before you go.
I recently hade a thoroughly enjoyable trip to Chile,and I wanted my camera gear to be carry on luggage-so it HAD to be small and light.
Looking at my photos,it looks like I made the right decision,although there is one lens I wish I could have taken with me.
Even if I'm shooting locally,I still have to decide what I want to shoot-my 35mm gear is heavy enough,add to that a Mamiya C33 body...ouch!!!!!