it depends on the strength of the reductors Mg and Sn.
--> Mg is a stronger reductor then Sn ( compare standard electrode potential -2,37 V with -0,14)
in general when the difference between oxidator and reductor lies between -0,3V and +0,3 V an equilibrium occurs (the case with Sn) and when the difference is bigger then +0,3V the reaction will be completely (case with Mg)
--> Mg will give a better reaction than Sn indicating more bubbles of H2-gas.