I'm no expert, but I think I can find a few reasons.
1: Crowd Control. Does your team have enough CC without you? if so; Soraka is a good choice. If not, Sona will be better in teamfights.
2: Your ADC. Do you have a mobile/safe ADC? Then Sona's gonna suffice, since you don't need to heal as much. Soraka has no damage compared to Sona, so a safe ADC will mean she has more mana and time for poke, thus helping more in lane than Soraka would be able to.
3: Their ADC. If they have Caitlyn, playing Soraka would pretty much be a death sentence. That ult is not something you want to be Soraka against. Lucian, Vayne, Kalista and Ezreal are pretty bad matchups for Soraka too.
4: Their support. If they have Thresh or Leona as support, Soraka will be able to save her ADC better than Sona would, while Sona is more effective against the likes of, say, Taric, Alistar, Zyra perhaps, and maybe even Morgana.
5: The rest of your team. If your team is relatively squishy, Soraka will be able to heal them for more and keep them up for longer. Of course, Soraka is also good alongside tanks, surprisingly. If your team already has decent sustain, say Olaf, Lee Sin, Fiora, Irelia ect. Sona may work better in the team.
6: Last, but not least in any way, The enemy team. If they have a powerful assassin like Katarina, Fizz, Yasuo or Akali who can quickly and easily jump in and out of your team, and possibly take someone while in there, Soraka will be a horrible choice. Especially against Katarina. Soraka can't build defensively due to her W, and as such, Sona would be a better choice by a mile since she can build a lot of health and not be punished for it, unlike Soraka.
Long post, I know, but I think these are kinda useful to keep in mind.
Edit: Ah, sorry. Didn't catch the part about "Normals Blindpick."
I'm gonna leave the tips here anyway, because they might still be useful.