I'd say Koscielny is marginally better but there really isn't much between Vermaelen and Mertesacker, and hence all three of them. This season I think they've all been guilty of some poor mistakes and they've all also played a few games where they've been very solid, and it all averages out to a pretty average central defence - which for the current poor standard of the league, still gets us a competitive defensive record.
Vermaelen gets a really raw deal from the fans and Mertesacker gets off a little lightly. It's becoming fashionable to take down the big names so maybe we can expect a disproportionate backlash for him when we realise that Vermaelen might not be the worst defender that has ever played.
Koscielny has put in a couple of good performances recently and when he gets a solid run of games under his belt I think he can be brilliant. But between Vermaelen and Mertesacker, I don't really mind who plays. Since we've had two clean sheets on the bounce maybe we can persist with the current pairing.
