There are a couple of key differences between their techniques. Wendy uses head voice and has very strong, robust support all the way up to A5/Bb5 and occasionally produces operatic resonance. Her connected head voice goes all the way up to G#6 (a feat even some of the best vocalists can't achieve) Sieun however uses Falsetto and hasn't shown that she can produce a true head voice. She is relatively relaxed in this register and has a huge range. I love both singers but Wendy not only has head voice support comparable to some of the greatest pop singers worldwide but has maintained a powerhouse technique for over a decade.
If you want to make a video compare Wendy to anyone, please, at least choose people who are at the same tier as her, by which I'm talking about someone like Taeyeon, Bada, and Eunji.
