I'm going to put this "Zyra vs Sion" thing to bed.
Okay, firstly, @TeemoHunter1337 you are correct in that Zyra's passive does have greater damage potential than Sion's. The original poster is mistaken in that regard, no doubt and I agree with you on that basis.
@Kowe_The_Ewok: The example of Sion's passive to Zyra's should never have been made comparative, in terms of damage and kill potential, but rather contrasting - as in how Sion's passive compliments his kit and his role in League, while Zyra's however, doesn't synergize with her role at all.
's passive forces the enemy to kite back from him or use cc on him. As a tank and not a sustained damage/burst mage, that's a win for him. Sure it does way less damage than Zyra's and definitely gets less kills, but if even one single cool down is blown on Sion after he's already dead, that is hugely impactful and serves the role he is meant to fulfill. Sure if Sion had Zyra's passive he would get more kills with it, but it's not about which is better as an individual ability. If that were the case then Zyra's wins hands down. However Zyra's passive does not in anyway support her game play, as a passive should (and Sion's does for him).
, simply put, doesn't want to die, and rarely should be in a situation where she does so. She wants to keep putting out sustained damage with plants and burst with her spells. They all have long range and with a Rylai's (which I always build on her whether mid or support) she can kite for days, not to mention some great CC already without it. It takes a particularly obnoxious assassin and an epic fail from my team's peel to blow me up in a fight as the mid laner, if I'm as support and get hard focused the enemy team has forfeited the fight anyway by leaving my carries unmolested. And so for what her role is her passive is VERY unimpactful, she would be better suited to one that supported her game play (and to a lesser extent, for
too, but that's a whole other subject).
For example (and right off the top of my head), give her bonus damage to a target 'revealed' by her seedlings OR dealing damage to a target 'revealed' prolongs your team's vision of them. Call it "The Happening" if you want the LOL's, or something else. That's why Riot have a champion development and design team though. point is thought they can do a lot better for Zyra.
To summarize: (TL;DR) Comparing them individually, yes, you win,
's is better but that isn't the reason why she should have a different one.
's passive works for his role in the game (to tank enemy abilities for his team and force them to relocate);
's does not for her role (to prolong her ability to deal repeated damage with plants while ccing the enemy).