The pekin is easier to butcher out sooner, which means less of a food bill. Compared to chickens, ducks are veracious eater. If you have the ability to allow them free range your billfold will thank you. Mine will wolf down anything imaginable. If you can put a bug zapper where they can get under it they will thank you as well. The little boogers are up all hours of the night waiting for bugs to fall. Same goes for a security light on a pole. Free range after they are turned loose, I only give them cracked corn as feed goes. It is up to them to hustle up their vitamins, minerals and protein,,,,,, which they do well. They are much like hogs and will eat about anything you throw their way. They also love to stay in the vegetable garden. I plant a lot of extra anyway since the deer help themselves.
Generally speaking the rouens I have raised are about the same size they just take longer to get fully mature. I like the pekins because the white is easier for me to pick. They also are plumper than other breeds earlier so they can be butchered before fully mature and have the same carcass as many mature breeds. When he goes in the oven cut slits through the skin on his breast to the fat. The fat will run out as he cooks basting the breast. This makes for a crispy skin and non greasy flesh. The pekins will just lay their eggs wherever. So if one intends to hatch you have to walk around their main area and gather them out of the yard. Unless hatching I just leave the eggs in the yard for the crows to get. If you have a bunch of crows around odds are you will have few hawks and owls.
I have raised them in 30'x30' pens on the ground with full feed rations. The pens are stripped clean of any vegetation and turn into a muddy mess from their dabbling. I raise call ducks, mandarins and wood ducks on wire and they do very well. So you can really do them any way you want, it's just an economic factor with the feed.