Taylor and it's not even close. Katy isn't America's sweetheart, America just loves her quick little singles. Other than that, Americans don't pay all that much attention to her (OMG, RECORD-BREAKING SUPERBOWL RATINGS!!!!).
Let's see... Taylor's best selling album in the USA have sold more than all Katy albums combined, she has sold more singles than Katy in the USA and also have earned more by touring. Taylor Swift is actually loved in the USA, Katy is not, people don't care that much about her as an artist - see the Superbowl. They just love her bops..
Considering that debut Taylor, Fearless and soon to be 1989 outsold Katy's combine US album sales with ONE Taylor album the answer is obviously Taylor Swift