I've heard their both great layers. My plymouth rock mix is a great layer. I have buff orpingtons coming with my order from ideal because of all the good feedback I've read about them on this site!
Rocks and Orps are about equal in egg-laying capacity, which is to say, they're both only average shakes at it but lay a nice brown egg. That matches my personal experience as well, although in egg size, generally, the Orps win, IMO, which differs a bit from the chart. Rocks may lay just a smidge more frequently because Orps go broody more on average. Not a lot of difference, though. They're both nice breeds.