And now for something completely different.
I also own both kinds of packs and have used them extensively over the years. And I appreciate Ben's reasoned arguments for the Top Loader. In fact I agree that the panel loader is slightly heavier and has more possibility for leaking.
However, I have had several occasions with a Top Loader where, in spite of my best effort to pack what I thought I would need on top, I needed to get something out of the bottom of the bag. This invariably happens during a rain storm. Of course I open the pack and the liner and plunge my hand down in the darkness fishing for the item. When I can't locate it by touch or I can, but can't get it past something else on top, then I end up emptying everything out of the bag in the rain and then repacking.
Since I switched to a PaneL Loader I have never had to unpack in the rain. Just a matter of zipping open the panel and grabbing the item needed and zip closed. I know there are problems, but for me the convenience is worth the extra wt.
But probably the best advice, and here Ben and I do agree, is Pockets–lots of pockets on the outside can keep you organized Top Loader or Panel.
-Mark