Idols typically aren't singers, they are performers, they can dance, they know how interact with the fans, interact on variety shows, they look pretty etc... actual singing ability fluctuates wildly, because thats not the main purpose of the idol. I would say SM is stronger in this field. As they seem have the reach to find rare singing talent and train them and then get them PS to make up for any looks deficit.
NingNing was a talented singer from a competition show in China, and SM were actually able to get her to come over to SK. And did some slight "adjustments"
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Similar with Winter, they probably thought she was too good a singer to pass up.
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