I would go with the khaki Campbells. We have two so far and they each lay a nice large egg every day like clockwork. I think they were about five months old when they started laying and for the last three months they haven't missed a day. We're thinking of getting a male and hatching out some babies in the spring
Campbells; white or khaki - they'll lay you a big fat egg every single day until winter. At that point they'll start to miss the odd day - that said my white Campbell started laying in January and has only missed a couple of days in may/june when my other ducks started to go broody.
Campbells and Welsh Harlequins breed true. WH were originally called honey campbells. Golden 300 and white hybrid layers do not breed true. I have Cambells and WH. WH are bigger. The female WH are as big as my Campbell drakes. Both are nice tempered ducks. That lay white eggs.