Well let me just explain the good and bad points of each of them

Sunny- sunny is very energetic when she dances, connects to the song and its all over her face, she can be creative with her dances. Her flaw is that she may try too hard to dance well

Seohyun- she does the dance the way its supposed to. Her flaw is that she doesn't connect with the song, and she isn't very creative

Me is Seohyun .. well Tiagih was right that sunny is energetic, connects to the song and so on and I love sunny too,, but I disagree in Seohyun.. because,, well yeah she dance the way it is supposed to but in her own way (if you know what I mean).. as you can see when you watch Seohyun you can see she put feelings in the steps and she adds face expression in them and she's very energetic too. Well according in what I see in youtube that people is saying or according to what I've watched.. Seohyun is next to the 4 best dancers (Hyoyeon, Yuri, Sooyoung, Yoona) because I've watched ones in their dance battle (maybe?) that Yuri is sick and then according for them to be complete they chose Seohyun to replace her.. MY OPINION ONLY DON"T TAKE SERIOUSLY

Seohyun

i think Sunny is one of the worst. totally boring, she's stiff and she also makes mistakes. doesn't show any energy. at least Tiffany tries

Seohyun is very boring to watch. She's emotionless. Sunny is far more creative.

Seohyun's a pretty good dancer Sunny's dancing isn't anything special