Taylor has written some good songs, isn't really much of a good singer and cannot dance.
Ariana doesn't write (to my knowledge? Does she?), is a good singer with a lot of potential and cannot dance.
I'd say it's a tie. Some of Taylor's lyrics are a bit cringe-worthy and repetitive, while Ariana's enunciation isn't the best and her belts sometimes sound kinda weak; especially evident on her cover of Emotions - one has to compare it to Mariah's fuller sound.