Nami or soraka?

Yes

They are completely different champs, but I'll do a (hopefully) quick breakdown:

: LOTS of heals, heals, heals, and more heals. You better stay in the backlines or else you're screwed, people focus you like a second ADC, and your peels are pretty limited as well. IMO she's kinda idiot-proof (as in even bad ADCs can do well with her), so is a pretty safe pick unless enemy picks a fishy all-in support like Blitz. Probably won't see play in competitive, but she rules soloQ like a queen. : Literally the jack-of-all-trades support, even though your engage is a LOT stronger than your disengage. Also for the awesome ult that's actually a great engage/disengage tool. You're more focused on poking and engaging, but is still a backline support. Like me, she NEEDS a good ADC in order to do well, even though you can pick her against anything. More likely to see competitive play, but soloQ is kinda fishy (no pun intended) unless you got someone to kick ass with you.

Up to you to decide.

Depends on what you're looking for.

Nami

They're both good. I'd say Nami is more idiot-proof if you can land your Q. But I like Nami more because she isnt completely helpless alone, unlike Soraka. I've solo killed carries with her.

I'd choose them based on...

If my adc is terrible: Nami If my whole team is terrible: Soraka

You want your team to never die? Soraka. You want your enemies to never move at full speed? Nami.

Nami carried me to Gold. Nami is just awesome. Her ult is very very versatile and strong when combo with her Q for CC chain. Her heal actually does decent damage. Her E slow is op for chasing. She rocks