I found an internal frame backpack far better for me--it moves with me, while external frame packs tend to zig when I zag and pull me off balance. You'll have to try both out to determine what works best for you. Do this in the store or at home with a fully loaded pack, preferably on ramps and going sideways up and down stairs to simulate uneven ground. With a one-story home, I almost wore out the three steps down to my garage! Do lots of sudden stops and starts and quick swings sideways. Like so many other things, this is a YMMV (Your Mileage May Vary) issue. Do note that some packs ride closer to your back than others, and there is a tradeoff between ventilation for your back on hot days and the stability issue.

As balzacom says, the heaviest items should be (1) closest to your back and (2) just above the center of gravity of your body. (Which is why women need to pack their packs differently from men, since our center of gravity is quite a bit lower)