Neither of them are ideal really, Carra nor Coates are that mobile so putting them up against some top class strikers is, lets face it, worrying. Coates has shown he can read the game well, but when he get's caught flat footed it costs us and him because he's got no recovery in him, and he's prone to mistakes too.
Carra has the experience, Coates has time on his side and is great in the air, neither are what any of us would preferably like to see against top class players, but then, it's hard to keep a great centre half as 3rd/4th choice, you just have to hope that the few good performances they are capable of, come when it matters.