Oxygen is harder than sulfur, so phenoxide will attack a proton more readily than thiophenoxide. This leads to the pK a assignments above. Transfer of protons between various nitrogen-containing functional groups is a critical step in many biochemical reaction mechanisms. The equilibrium lies to the right. Briefly but clearly explain your reasoning.
Any equilibrium favors the most stable side.
Note that the arginine is accepting the proton on the doubly bonded nitrogen in this reaction. Resonance increases the electron density to this nitrogen, making it more basic. Ammonia does not possess this increased electron density, so arginine will be a stronger base than ammonia. This same resonance delocalizes and stabilizes the charge of the protonated arginine. This suggests that protonated arginine is weaker acid than the ammonium ion.