They do feel very different on your back. Duplex is stiff "external frame" and Wraptech feels more part of your body. Duplex can carry more comfortably but is a bit heavier. WrapTech packs will tend to "Barrell" when fully loaded. The WrapTech suspension is in my opinion the absolute most comfortable suspension of them all. That said I sold my T3 (wrapTech) and purchased a DT3 (Duplex) for more versatility. I did put the WrapTech Lumbar Pad in the Duplex belt for greater comfort.
Remember that The Late Season has been redesigned (Late Season Legacy 3400cui). It has some Pals on each lower side to attach any of the belt pouches and has had some loops added for pods or more pockets. I think the biggest bonus of the two is that the Late season can now be ordered with 24" stays. That alone would make my decision very easy. Both great packs but The Late Season has gone to a whole new level.
Like I stated above I reciently went with the Duplex system for versatility. I am a huge fan of the Timberline packs because of the pockets and ability to organize everything. Now if needed, I can just order a different Timberline bag, Longhunter, or just use some pockets and pods on the frame. Tons of versatility. Best of luck with your choice.