Many people prefer Barrier over Heal in ARAM, but by doing this, they disadvantage themselves before the game even begin.

I wrote some simple calculations to explain to you why you should rather take Heal.

Heal : 120 - 20% (due to the reduction in Howling Abyss ) = 96 HP if used on one person

If used on 3 people : 96 x 3 = 288 HP
If used on 5 people : 96 x 5 = 480 HP
If used on 5 people with reduced healing on themselves ( Ignite, etc): 480 - 50% = 240 HP

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Barrier : 175 (with Summoner's Resolve )

Barrier's cooldown is 30% lower than Heal's CD, so let's say that Barrier's output is higher than what it is in order to make the comparison correct accordingly to the two different CDs.

175 + 30% = 227,5 protected HP

Heal : 345 - 20% = 276 HP if used on one person

If used on 3 people : 276 x 3 = 828 HP
If used on 5 people : 276 x 5 = 1380 HP
If used on 5 people with reduced healing on themselves : 1380 - 50% = 690 HP

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Barrier : 475 (with Summoner's Resolve )

475 + 30% = 671,5 protected HP

1) With Summoner's Resolve , Heal can passively increase your maximum HP.
2) Heal can help you to get assists.
3) There are several healing reductions and you need to heal several people for Heal to be efficient, but even if you don't use Heal with its best potential, it can still be more efficient than Barrier.
4) It can be great for your team's morale to suddenly see several green lights radiating from your champions.
5) To avoid the healing's reduction from cumulating two Heal, it would be best not to take 5 Heal per team.
6) You can reduce Heal's cooldown with Eleisa's Miracle - and if you intend to buy this item, then it's also a good occasion to take Clarity.
7) Barrier only lasts 2 seconds, so if your shield isn't broken by the enemy, it's often a big waste. On the contrary, Heal can't be wasted unless you make a mistake.
8) As you could see with the calculations, a perfectly used Heal is superior to two Barrier.