The Heartseeker skin line is definitely a sexually-driven one and I don't feel like Orianna really fits that, which makes her a great candidate for the Sweetheart line. Sona is definitely a more sexualized champion and therefore would fit better in the Heartseeker line.
i wish they did a better job on sona's skin... this just looks like they did no work on it.... her aura's have hearts in them, her ult has some hearts, and then they recolored her a little bit.....
"Heartseeker" fits Orianna's character more than "Sweetheart". Here's an excerpt from her lore:
"She tries in earnest to fit in with those around her. However, no matter how hard she tries, Orianna can never be human and there is always something unnerving and alien about her."
Does that excerpt not imply that Orianna is seeking for a human heart? Not only does she fit the title, she also fits the implications of the title, albeit slightly less well. Orianna is searching for face-to-face interactions with humans, a very human action, and one of the precursors to love. Furthermore, judging by her in-game quotes, Orianna is very active and, perhaps most importantly, deadly.
"I know what makes them tick. I know how to make the ticking stop." -Taunt
Yeah. Not a "Sweetheart" at all.
I personally don't think either fit the Heartseeker line. Sona is actually an innocent sweetheart herself and there's nothing overly sexual about her outfit except her cleavage, when then again isn't Vayne/Varus/Ashe level.
Heartseeker Jinx FTW!
It's not really sexually driven.
It's supposed to resemble Eros or Cupid.
The problem is they can't actually make the champions fully naked, not only because it would make for some lawsuits, but because it would be hard to recognize the champions if the model was completely different.
Also, even if that was the case, Sona's skin is not sexual at all.
i'm so glad it isnt heartseeker 
, Quinn AND VALOR 2 champs in 1 re-rework/revert to previous kit confirmed?
to be fair, all previous heart seekers had bows of some kind.... or flat out marksman, then they added more with annies and now sonas and ori :/ I/m fine with orianna's but sona has a freaking ultimate now... why would they sell skins after that.... usually ult skin means there won't be anything new for em for A LONG TIME...
[slayer-jinx-unamused] not feeling it
Kateykhaos dropped an answer on the PBE boards
The name Heartseeker/Sweetheart is determined based on the personality of a champion, as opposed to the weapon they wield.
c/p from another reply on this post - We took in the personalities of both Sona and Orianna. Sona is more passive, and more, well, sweet. Orianna is a robot (does she even have a heart, and if not, is she seeking one?) and would definitely be more aggressive and more of a killer-type than Sona would.
I don't feel like Orianna really fits that
