Longer range poke along with it being more reliable
Sona's Q out ranges Nami's W, but it also does pathetic damage. If you want to do any damage at all you need to AA, which has a much shorter range.
When sona pokes she doesn't have to give up her shield/heal ability
Neither does Nami, it bounces back and heals you.
Movement speed buffing multiple allies at the same time doesn't require a waste of an aoe skill
That's an advantage. Nami effectively has another use for her speed boost. Sona's just left with a crappy speed up all the time.
Her ult is easier to land
You don't have to land Nami's ult. Its main use is to disengage, which means if the enemies need to waste time dodging it's done its job.
Nami is just a superior Sona right now. Picking Sona over her isn't justifiable right now and the fact that they're the best 'advantages' you could think of shows that.