Sedna is falling behind a little bit, but it's still too early to tell.
End of Kitkun's quote
Sedna was good in beta, yet got buffed the most. 
I am not a great sedna player, but the people i've seen good with her just wreck people.
I think dark side is winning because A: TB is sitll a little overpowered and B:More good beta players are on the dark side.
I think if gpg balanced the game based on what won most, what killed most, and so on, we'd be pretty screwed.. cause that guy who gets 25 kills 0 deaths every match and plays all day, and ALWAYS plays a certain demigod, can considerably sway the results. It's just because he was good, and liked that demigod because it fits his play style.
If you've ever seen games with leaderboards, sometimes the #1 guy has like 2000 kills 50 deaths, which is better than the near 3 guys combined. So the whole does not always balance out what the favoritism of players can do.
I think Demigod is greatly balanced demigod-by-demigod with the day0 patch except maybe the TB having a SLIGHT advantage, but nothing player skill can overcome at all like earlier. Some players are just good at using a certain demigod that they can make them seem overpowered compared to how you use yours.