Covalent bonds are stronger than metallic bonds.
In a covalent bond, atoms are pulled closer together because the bond is stronger. The electrons are being shared in a covalent bond, but due to electronegativities, they may spend more time around one atom (polar).
In a metallic bond, electrons form a "sea" of electrons. They are not pulled to one atom or another. For this reason, metals are good conductors due to electrons not being held closely to an atom.