This is like choosing the "better" option between blindness and deafness.
True that they are both mediocre "vocalists" overall but this thread is about if you had to pick who is better technically in their live performances, who would you pick out of both of them?
Honestly it could go either way, taylor is consistently very average. Katy is also pretty average but she sometimes has great vocal moments that Tay can't compete with and sometimes has disasters worse than any taylor live performance.