That might well be true for some pairs of nucleophiles; certainly hydrogen bonding to the nucleophile cannot be ignored. In dipolar, aprotic solvents, the familiar order of nucleophilicity, iodide > bromide > chloride > fluoride, is reversed. The only way to know for certain is to look at the rate constants for particular pairs of nucleophiles in the specified solvent. Thiophenoxide ion is a stronger nucleophile than phenoxide, as judged by their values of n the nucleophilicity value in methanol (Lowry and Richardson, pp. 333-334, Mechanism and Theory in Organic Chemistry, 2nd ed). However, if R is alkyl, the story might be a little different, and I don't have any information handy on polar, aprotic solvents for these two classes of nucleophiles.