Both for 歌う because Taeyeon has a natural,stable voice and can control it while Jessica is trained one and can sing higher notes.
For who is もっと見る popular, I don't really know because snsd is really popular, so i can't really decide since Taeyeon is 人気 cause of her dorkness and her 歌う while Jessica is 人気 cause of 歌う and beauty.