This question lies at the dichotomy of an artist; from reading tvtropes page on Performer vs Technician - I realized tat Taeyeon is a perfectionist Technician while Sica is an emotive Performer; it rlly crystallized further during her time on Sisters who make waves while she was guiding her group in dance tutorial n stage performance; her group weren't technically the best but Sica taught/guided the group to dance/sing/perform in my opinion to evoke feeling in the lusteners/audience/judges; while her group dint win every round but Sica herself wuz robbed in the finals cos they couldnt/wouldn't let a Korean-American win tat yr n a Taiwanese idol from the 90s/00s Cyndi Wang won instead.
The winner may have technically been a better singer but the stated goal was never about the best singer imo; but alas Sica lost/got 2nd place but she did go on several or other China variety shows baaed on her results/strong audience rapport/reception(i so believe) anyways i do think taeyeon is technically better as a singer than Sica but i hv always felt more from Sica(even before I bias-ed her if tats a turn of phrase). It wasn't until I saw a clip of Taeyeon performing live in concert the song "a walk through memories" or watever the title is tat I felt emotionally moved by Taeyeon's singing. I consider this my favorite Korean language song over any other kpop song, but only as performed by Taeyeon - others performances fall short in my eyes even the original singer/writer/composer of the song - Kim Jong Wan & NELL, I just feel his emotiveness n singing/performance below Taeyeon's(taeyeon performed this song at least 4 times/eras - 2 concerts/different tours i know of, on Begin Again n also a recording on one of her albums. It's one of the only songs not originated by her or SNSD tat I know of with tat distinction. Tat song is overall very popular in Korea gp I think. Maybe not understanding korean skews my perception of NELL/Kim Jong Wan's sknging/performing it but I felt moved from the first I heard/watched Taeyeon perform. Anyways just my $0.02 on who's the better singer(my bias in SNSD is still Sica tho)