Seriously, 10% of MAX health? atleats make it current health, and why does it cost health AND mana? choose one, not both Riot.
Currently there is NO situation in which anyone would choose Soraka over Janna or Sona...why? she runs out of health so fast and her CC/poke is WORSe than both of the other two.
Soraka's mechanics are more than Sona's and Janna's, imo. Really their harass is much harder to use. And really, Soraka is a bit of an in between for Sona and Janna protection-wise. Sona with her 10 second CD on heal is not gonna be useful in sustaining teamfights much, and Janna's shield isn't going to sustain outside teamfights much either. Soraka's low CD gives her a bit of an odd inbetween. She can kinda sustain teamfights and longterm.
Soraka's silence is very strong and her heal + ult does more than either of those two can. Soraka is one of those champs where her kit's power level seems low until you play her a bit and can appreciate how good those clutch ults or silences are.
Soraka is far better than a Sona in teamfights
You have to build health regen on Soraka. I love playing her on ARAM, moreso than Sona. I don't even like Janna. Soraka's aoe harass is really fun and she heals like crazy. A typical ARAM game with Sona : 30k healed. Soraka : 60k healed. I played her maybe once or twice before her rework but I can honestly say afterwards I enjoyed playing her enough that I bought her. She is one of the most fun champs to play on ARAM imo.
Well, Soraka's heal is 4x of Sona's and she has a silence and higher base damage. Not to mention Starcall's heal is roughly the same as Sona's self-heal as well. Meanwhile, Janna grants the ADC an absurdly high amount of raw AD for a very long duration via her shield. Sona used to give AD, then she got nerfed... Janna also has that permanent 1000 range MS aura. When you think about it, Janna is basically a better version of pre-nerf Sona, but with better pre-6 CC and no pre-6 heal.
In usability ranking, it looks like this: Janna > Soraka > Melee Minion > Sona
Warmogs on Soraka wins always. Really though, all you need is the health regen and heal increase (spirit visage) and she has incredible power http://matchhistory.na.leagueoflegends.com/en/#match-history/NA/40109297?champion=16
