According to Fajan's rule, smaller the size of atom higher will be polarizing capacity and so the covalent character will be larger.
Here, we can find that at higher oxidation state like in Sn , the size of cation Sn+4 is quite smaller than the Sn+2 cation and so higher oxidation state of the metal will have more polarizing power and so it will be able to neutralize the anions in greater ratio and so they will be more covalent than the lower oxidation state cations.
So, SnCl4 is more covalent than SnCl4.