It won't be very long before you know for sure, but I still see one of each. As they get older, I am betting the differences between the patterns will become more clear. The Plymouth Barred Rock will have distinct, crisp black and white barring compared to the Cuckoo Maran which will have a softer, lighter, barring. And the coloring of the legs should become more distinct as they get older.
I have one Cuckoo Maran and she is the lowest on the pecking order of 10 chickens. She is just a little bit smaller than all the other birds I got. Very sweet, but definitely lowest on the pecking order.