You are absolutely right, they serve the same purpose. They main difference is that the Quantikine kits contain pre-coated plates (coated with the capture antibody ), whereas with the Duo kit you need to coat the plate yourself, usually overnight. So the Quantikine kit saves you some time and has arguably 'better' coated plates but I usually use the Duosets and they work perfectly fine!
So DuoSet kits apparently are cheaper than Quantikine, since these ones are already prepared and they save time, because it's not necessary a extra step?