Rocks usually grow faster in the early months than Orpingtons do, so you might want to consider them. I think Wyandottes and Rocks are pretty similar (except comb type and colors), but I haven't actually had Wyandottes so I'm not positive.Me and our family also want to start raising our own meat chickens and we were thinking about getting Orpington‘s or Wyandotte’s because those are pretty big breeds.
Mature size only matters if you are raising them all the way to that size.
Otherwise, size and amount of meat at butchering age is what really matters for a meat bird.
You could buy several of each kind, raise them together and compare weights at various ages, then compare how meaty each one is when you butcher it. Then buy more, for breeding purposes, of the one you liked best.
One breed (like Buff Orpington) might perform differently if you buy them from one breeder or another, or from one hatchery or a different hatchery. So trying out the ones you are considering will tell you how the ones from THIS source perform, which is more accurate than looking at numbers telling what they are supposed to do.