Lustrate is better than benediction, here is why:
Benediction has a very long cooldown so it can only be used 1 per match and you will almost never get the full effectiveness from it due to the delay in it's cast. Since it has such a long delay to activate a lot of times your target will be dead by the time it goes off and to top it off it will still burn your cool down. To counter this you have to use it earlier than you would normally like, if your target is getting focused hard you'll need to fire it off when their HP is around 25% or 30% just to make sure it lands and doesn't get wasted. Firing it off early like that wastes 20% or 25% of the actual heal as overhealing.