who is a better support both supports you mentioned are good for different things. if you are facing a poke support (imagine a sona support on the other side) soraka will be better because she has a lot of sustain so even if your adc gets poked down you'll just heal that back up if you are facing an engage support on the other hand (imagine an alistar) then janna will work a lot better since she'd more adept at disengaging the alistar with her tornados