In my experience, as a mountain biker of 20-years, you want to seek weight minimization and distribution.
A heavy pack (assuming a non-trivial ride length) is going to cause your lower back to hurt, make you hotter (back can't breathe), and put additional stress on the palms of your hands - all of which become more problematic the longer you ride.
A heavy load on the back of your bike (rack or carrier) will interfere with your front-tires ability to remain in contact with the ground - making climbing and steering more difficult.
A heavy load on the center of your bike and lower down will cause your bike to feel more sluggish - harder to turn and make for a less enjoyable ride.
First and foremost, ask yourself what you can get rid of. Consider carrying patches + 1 - tube instead of 2 tubes. If you know you will have streams in the area, get a good filtration bottle to lighten the H20 load. Can you learn anything about the vegetation that will allow you to eat local plant-life to reduce the food you carry? Do you have high-end clothing that is both light and protective? You weren't really specific so these may be off base. The point is to be really creative on how you can carry less. No matter where you store it, less weight will always feel better as the ride gets longer.
Also, you might consider a bike trailer. There are some really nice ones (example) that minimally interfere with your ride and will do a much better of carrying your gear comfortably than trying to put it all on you or your bike.