Just to second all the advice already given here really...
We have:
* Both Khaki and White Campbells - definitely the best layers - an egg a day for the whole laying season - hardly worth bothering trying to eat
* Silver Appeyards - reasonable layers and just about worth eating
* 'Aylesburys' AKA generic white farmyard duck. We do get some eggs, perhaps one every third day on average but the make fantastic eating and that's what we keep them for.
* Cayugas - these are new to me but seem to be better layers than the aylesburys, maybe an egg every other day or better. Not eaten one yet but they look a good size.
So in summary, for laying I don't think you can beat a Khaki Campbell but don't get your hopes up for getting much eating. I think if I was less duck-obsessed or had to cut down numbers, I'd keep as many KC as I could fit in and then buy in some day-old aylesburys every so often to raise for meat.
If you do hatch some, be aware that you want a fairly high ratio of ducks/drakes - I aim for at least 5:1 so you will have to deal with a load of unwanted drakes!