Keeper league or re-draft? If keeper, then definitely grab Johnson. How many WR's do you start? Colston and Burress should be solid. Coles is obviously the safe pick but Johnson has big upside. Personally, I'd take Johnson (and I did in my recent draft). It's a gamble but at least you have some other solid WR's to fall back on if he doesn't pan out.
If it is a non-keeper league, I'd keep Coles. Rookie WRs rarely have huge seasons, but Coles always puts up very good numbers. Roy Williams will continue to be the guy in Detroit this year (and maybe into the future too -- he is pretty freaking good...).
You have two solid starters in Colston and Burress, so why wouldn't you take the filer on Johnson? You know what you got in Coles, but Calvin has the better upside.
