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.