Why prefer $MgSO_4$ over $MgCl_2$ in recipies
My guess is its the solubility properties of the anions.

Chloride ions can precipitate with some metals that might appear in a complicated buffer or medium. And yes they could even compete with enzyme binding.  Sulfate will remain in solution with just about anything.  

There are exceptions, but if you're making a buffer you're probably going to do better if you use the general solubility rules.