i know there are pros and cons to both, but is there any difference between tubular frames and flat bar frames? i'm get an internal frame for sure, but which would be more comfortable and better at load distribution? and should the heaver gear be loaded at the top of the pack or bottom?

I'll let someone who is more of an engineer address the issues of tubular vs. bar vs. other frame materials.

Buy the pack that fits you. That's the first key.

Pack the heavy stuff close up against your back and low in the pack. That way it puts less stress on your shoulders, and allows your hips to manage most of the weight.

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)

I think the person asking the question is talking about internal frame packs - those with tubular stays vs flat or square stays. It is not real clear what they are asking.

In general, round tubes are stronger for the same size and mass as a square equivalent.

I though most internal frame packs either used a flat plastic sheet, stays (thin tubes, either fiberglass or metal, or a metal bar about 1-2 inches wide, right down the middle. All these attach the top of the pack to the bottom (hip belt area) of the pack. Some packs us a combination of the above.

Internal frame sheets and bars are shaped to fit your back and can get flattened or out of shape and make the pack uncomfortable. I am not sure that applies to stays. I think how you use (abuse) your pack may determine how long the internal frame remains good. For example, if you regularly sit on your pack, that could reshape the stay, back sheet or bar. I think if you regularly overload the pack (more than the recommended weight for the suspension system), it would also degrade more quickly over time.

External frames carry differently, so you have to pack it differently. If you are on trails, you can pack the weight really high, which although unstable if scrambling or off-trail, puts the weight over your center line, so there is less of a "lever arm" to pull you back or make the weight effectively heavier. I notice that most people with internal frame packs lean forward to compensate for a center of pack weight that is farther from their centerline. A lot of this is a moot point if carrying 20 pounds or under. At 60 pounds, you will feel the difference.

Either an internal frame or external, I distribute the weight differently depending on if I am on trail or off. And at 50-60 pounds, regardless of pack type, a pack throws me around simply because it is such a high percentage of my body weight.

An internal frame does hug your body more. When internal frames first came out ski-touring quickly adopted these packs as easiest to ski in.