In a developer Potassium salts will give marginally finer grain than Sodium, this is most noticable with warm tone developers where it's the fineness of the grain that alters the warmth.

The differance in solubility between Sodium Carbonate 215 g/L (20 C) and Potassium Carbonate 112 g/100 mL (20 C) is quite marked, hence why Potassium Carbonate is used in commercial liquid developers.