I put the weight on the rear simply because it is easier. There is reason to believe that the front may be a better choice, but I haven't ever tested this theory. I do know that, even with only a moderate load on the back (~15 lbs), I get kind of a squirrely feeling taking corners while accelerating over 10mph, like the front wheel is trying to wash out to the outside of the turn. This "understeer" feeling is VERY disturbing on a bicycle. I don't appreciate the degree to which front panniers dampen steering, but it would be more stable.