Wait, wtf, why is Kayle's heal better than Soraka's? Like a Wookie on Endor, that makes no sense.
Lets break it down!
Kayle's range on her heal is 900, she can self target, it doesn't cost 10% of her health, it has higher base heal as early as rank 3, and it gives an 18-30% movement speed bonus for three seconds, and that even scales off of every 100 ap for an extra 7% MS per 100.
Comparatively, Soraka's heal costs slightly less mana, 60-100, starts out stronger than Kayle's heal at rank 1-2, has a CD that goes all the way down to 2 seconds, compared to Kayle's flat 15 seconds, and scales a bit better, Kayle's heal is 45% AP, Soraka's is 60%.
Problem is that Soraka's heal cost 10% of her health so unless you like.................Rush Warmogs...and then a giants belt or two, you have no way of consistently applying yourself outside of dropping Q's, problem there is Soraka's range is not incredible, and when she fires them out at max range they drop quite slowly. She's not useless until she uses Equinox, then she actually is pretty useless as she can't heal, she will put herself at death's door, her health is too low to risk going in to drop a Q, her equinox and ult are on cooldown...literally all she can do at that point is just go back. she's essentially dead at that point, lol.
Soraka's ult heal scales pretty poorly, whereas Kayle's ult just gets better with time because it blocks more as people get stronger. Soraka's ult heal does not keep up with the Jeffersons.
Overall, looking outside of just healing, they are pretty close. Equinox is an extremely powerful ability, it shuts down quite a few ults, and Starcaller (if that's what its called? Soraka's q) with Astral Infusion makes your lane have endless sustain. Thing is, Kayle's ult is way better, her Q is a lot stronger in a fight than 'raka's Q, I'd say Kayle's heal is better because raka's heal hurts her and she can't even heal herself, so having to have 2750 health+Warmogs really fucks up Soraka's support build badly, or else you end up essentially "killing yourself" by healing people who would for-real die, Kayle's passive peels off armor and MR, plus she's a lot better at knocking down turrets than 'raka.
and sometimes 
or 
in which case, she doesn't always have max CDR and even if she did Soraka can almost heal a target 5 times in the span of Kayle's CD. And most of the time the heal is the last thing Kayle maxes to, so it needs to feel impactful when she does put points into it.
as you have way more heals to work with (for Ardent) while if you went Kayle support she would only be able to get 1 person with Ardent because of her higher CD heal (not sure if Kayle ult counts for Ardent or not. I don't think it does).
