Actually you get one or the other depending on class, it is pretty unreliable seeing as the game has kill trades where the ability activates but you get no benefit from it because you die first leaving you to respawn with the ability on cooldown. The titan doesn't have range on their melee losing out to both warlock and hunter in the very common scenarios of cqc particularly with shotgun + melee follow up. While the titan class needs to kill with melee to trigger their ability the warlock just needs to hit you and gets the same shielding making them a better more reliable tank than the tank class.