I have both and have used both for hauling and hunting... I would echo what has been said above.
The bikini is lighter and slightly more comfortable, but for really heavy or awkward loads, the duplex gets the load. That said, I have never had a situation where I felt the duplex prevented be from moving or shooting a bow any worse than the bikini.
I tell people this.... If I were to only have one frame for all-purpose duty, it would be the duplex. If you have both, I hunt with the bikini and use it for first load out, but have my duplex with the cargo panel and a couple pouches ready to go in the truck or in camp for the heavy stuff.
Joe