Resistances are more efficient for making yourself melt slower than health on him. There's really precious few exceptions to this rule; Morde generates shield every hit and has spellvamp as his core to regenerate health every hit and his ult has massive inherent spellvamp; he absolutely swims in sources of temporary HP (his W will keep providing it no matter how CCd and he's a tough customer to burst down). Any MR and Armor makes all that so much more valuable.