Why even bother to pick Sona over Nami with this "Better Nerf Sona" trend

Because right now, Nami does everything Sona does, only better. These two champions are very similar in what they bring to the table.

-Movespeed boost -Lane poke -Lane sustain -On-hit AA buff -Aoe hard CC ultimates

-Movespeed boost - This one Nami gets on every ability while Sona has to invest in it. Also, Nami's will be stronger in most situations on allies because, while it falls just short in base than max rank Sona E it has triple the scaling.

Oh, and Sona's E is the most underwhelming ability in the entire game to level up. 10+1 % move speed per level, wow. That's, what? 3-4 move speed to tagged ally for 1.5 seconds? Karma gets 30+5% on her SHIELD that also gives MS and is always at 60% if cast with Mantra. Lulu gets +1 second on her MS boost with 2-3x Sona's base and 3x her scaling and can be used as a pseudo-hard CC.

Lane poke and sustain - I put these together because Nami does them both with one spell. And that one spell deals more damage than Sona's Q + empowered AA (not counting power chord) and has better heal base (but slightly worse scaling) than Sona's heal and shield COMBINED (those heal and shield can't even compare to A SINGLE AUTOATTACK in effective HP). And it can also hit two enemies or allies if positioned properly.

And since Nami gets 2 of Sona's spells for the price of one, she has another spell to use. And it is an AoE hard CC, instantly making Nami much more likely to survive tight situations or secure kills for her carry.

On-hit AA buff - Sona gets to tag herself and allies for a single AA boost, Nami gets to tag a single ally for 3 AA boost. Nami gets better base until Sona gets level 3 Crescendo and a slow on her attacks. Consider that Sona had her passive greatly nerfed to make room for this new buff. That shifted a big deal of her poke power from herself to her ally, who will waste it in 90% of cases in soloQ. And if Sona ever tries to make full use of her tag in skirmishes, she's dead because she just came into enemy assassin/bruiser's range in order to tag her bruiser with almost-melee-range aura.

AoE hard CC ultimate - Sona also gets passive buffs to her base abilities... too bad those buffs are pathetic and don't even compensate for nerfs she received and might as eel not be there late game (+30 flat damage and shield. Wow.). Only advantage Sona gets is near-instanct cast. Nami gets better scaling, triggering her passive, near-triple range, lower early CD, uncleansable CC followed by soft CC.

I love both champions, but at this point I can't think of a single redeeming quality Sona has over Nami in terms of what they bring to the team.

//================================================================================== EDIT: One of the big problems is simply how Riot is treating Sona post-rework. I was actually excited for the rework for some time. Oh boy, do I hate it now. It's like they look back at things they wanted Sona to be able/not able to do and are, like, "Fuck that, let's just nerf her". Just look at her Q and W power shifts for example:

Her Q gained an on-hit AA-buff effect. In order to make room in her power budget for it, aside from obvious removal of a permanent aura, she took a substantial hit to Q activation damage and Q powerchord. But then, they decide to nerf the Q on-hit effect.

Her W lost a lot of base heal value and scaling, but gained a weak shield and a missing %HP scaling. Reasoning was that they didn't want Sona to just keep her ADC's and her own HP topped in lane, they wanted more of a combat-oriented/clutch heal spell. And that'd be actually nice - BUT THEN THEY DECIDE TO CUT THE MISSING %HP RATIO IN HALF, but with a whooping 'compensation' of +5 to base heal value. Oh, and increase the manacost. So now your heal is crap regardless if target is at 95% or 5% HP.

I thought the purpose of the 'rework' is to shift power from passive parts of her kit towards the more active gameplay. But nerf things into which said power was shifted enough, and the so-called 'power shift' stops being a power shift, and becomes a straight-out nerf.