Personally I think Turner is our best CB, I agree distribution etc is not the best but he's more a blood-and-guts body on the line CB.
Him and Bramble together were excellent in the first few months of the season, and if they started next season fully fit as our first choice pairing, I wouldn't mind a bit.
If we did get Johnson, not sure which one would make way- from what I've seen of Johnson he looks more like Turner in terms of playing style, so I'd probably opt for Johnson and Bramble.