I think he's looking for external frame because of the air circulation. I have a friend who has an Atmos series pack (35 or 50, I forget which) and he says he really likes how the pack's design lets the air get to his back - said it reminded him a lot of how the external frame packs he used "way back when" stood away from his back.

Before irrevocably committing to an external frame, the original poster might want to look at the Osprey Atmos line. If he's at all partial to internal frame packs, they might just combine the best of both worlds.