Whether Covalent bond is stronger or not is determined by Fajan's rule. In some compounds covalent bonds are stronger and in some compounds ionic bonds are stronger.
Polarization refers to the ionization (complete transfer of electrons) and the charge on ions. Covalent bond refers to sharing of the electrons and reduction in polarization. Covalent bonds reduce electrostatic attraction.
In reality the compounds usually have some covalency between ions. Because when ions come close to each other due to electrostatic attraction, the outermost electrons in the anion are attracted by the cations.
Polarizing power = charge on cation / radius of cation
Deformability of anion = polarizability = softness depends on electron cloud distance and charge on ion.
1. Small cation size and high charge on cation will increase polarizing power.
2. High charge and large anion size will increase polarizability
3. Between two ions, the one with pseudo noble gas configuration (18 electrons in valency shell configuration) is more polarizable than that with noble gas electron configuration (8 electrons in valency shell).
When polarization is higher, then the formation of covalent bonds is stronger.
Na Cl is more ionic than covalent. Cu Cl is more covalent than ionic.
Al F3 is less covalent and more ionic. Al I3 is more covalent than ionic.
Hg (2+) is more polarizing than Ca (2+).
Iodides are more covalent compounds.