I don't have experience actually eating either orpingtons or wyandottes, but I have a couple of both in the growout pen right now, and the orpingtons are definitely more what I like to see in a meat bird. They're a much bigger bird, for one thing, with a fuller body and nice big legs. The wyandottes are lovely birds with a nice compact shape and pretty feathering (I have the blue-laced red), but comparatively there just isn't a lot of meat there.
The orps are also really friendly and and outgoing -- they're always out in the yard foraging and following me around looking for goodies. The wynandotte is more stand-offish and keeps to itself.
If it were me I'd choose the orps.