Many people seem to have a reaction when I point out that one breed is a better layer than another.
I can say that in general a Barred Plymouth Rock will lay 4 eggs a week on average in a year, whereas a Rhode Island Red will lay 5 eggs a week on average in a year. So I think I am safe in saying that on average Rhode Island Reds are better layers. Of course there are individual variations.
I put links to a few websites that show the number of eggs laid by each breed.
Plymouth Rock
http://www.mypetchicken.com/chicken-breeds/Plymouth-Rock-B85.aspx
http://www.cacklehatchery.com/barred.html
http://www.keepingchickens.com/plymouth-rock-chicken/
Rhode Island Red
http://www.mypetchicken.com/chicken-breeds/Rhode-Island-B97.aspx
http://www.cacklehatchery.com/rhodeislandrdpage.html
http://www.keepingchickens.com/rhode-island-reds/
IMO none of these sources are a reliable source for accurate information.
Walt