Carroll's proved that he's pretty fucking dynamite in the PL. Suarez hasn't, obviously. He's looked good in a poor league and against poor players in the WC. Obviously, Carroll is by far the safer bet for PL clubs.
If there's one thing that's been rammed down our throats in the last few years, it's that players who look special in other leagues aren't guarenteed to make any impact in the PL. Away from NUFC, I can think of the likes of Morientes, Crespo and Kezman.