I would wait until REI's 20% off member sale. If you buy the Back Packers at full price you will lose out on the savings you can get here:
I have a set of Back Packers and Sport Packers. You shouldn't need any more room unless you are carrying A LOT of stuff. They keep things dry if properly closed, which isn't hard to do. Make sure the flap on the outer pocket is properly closed or you may get water in it, and it will stay there.
One thing you have to ask yourself is whether you a one big compartment person or a lots of small(er) compartments person. I started out the latter with my first set of panniers. When I was forced to buy new ones, I was worried about the transition. Turned out not to be a problem. You can always incorporate individual stuff sacks or even Zip-Loc bags if you feel the need to segregate things.
As noted, critters can get into most things. (I stupidly left some bread in my tent when I went to visit a museum. When I returned a few hours later, I found a squirrel had chewed through the tent mesh, crawled inside and went to town on the loaf.) I would definitely not leave them unattended with food in them.
When is the 20% Members sale?