The Duplex is going to handle awkward loads a little better, especially if you don't pack correctly.
You have 3 bags and 1 frame to choose from, so versatility is better with the Duplex.
Weight savings and simplicity are the strong points of the standard Timberline packs and for a 5-6 lb pack they handle weight very well. I actually notice little difference between the two with heavy weight, but I take care when loading it up too.
I think it will boil down to each persons style of hunting.......
If you come out west for 7 days each season and only use the pack for that, then the Standard is a good choice.
If you're hauling out 15 plus animals a year, hunting in multiple states and hauling crazy loads, than the Duplex is a pretty hard option to pass up.
Taller guys looking for a load hauling daypack will like either model, but if that guy wants a "do all" type system and will head to AK for a sheep hunt from time to time, than swapping the bags on the Duplex is a nice way to go.