Internal vs. External Frame Backpacks

Which is better? External or Internal frame backpacks?

This is a personal answer to a question about comfort. I like external frame backpacks because they seem more rigid to me. When I walk, it seems the pack says firm.

My wife likes the internal frame packs. To her, the frame says rigid and is compact and softer.

I still don't know how an internal frame pack can be rigid. It seems that an external frame is wider and by it's nature is more stable.

Please let us know your opinions.