If you'll be in a place where the weather is uncertain, you should look at one of the LowePro bags that provide water protection. They have a thick rubber liner (sort of like a bladder) that zips completely closed, protecting your gear. I haven't used one, although I've been intrigued.
I have used some of their Trekker series and ones that are both a shoulder bag with a harness built into it. I was very pleased with it. I used it to haul around a fairly massive Rolleiflex SL66, three lenses, two hoods, two backs and about 10 rolls of film. Plus, I stowed my Sekonic light meter in it, too.