Carrie is by far more successful in the US, while Kelly is more known world wide. Carrie completely obliterates Kelly's numbers in the US, except for that one Kelly single that sold a lot (hey, guys, success is not measured by ONE single!). I don't know why this is even up for debate.
So if success = being known outside the US, then yes, Kelly is more successful (although it's not even fair competition because country is a local genre).
If success = having the single that sold the most, then yes, Kelly is more successful.
If success = higher album sales on average (in the US), more Grammys, higher grossing tours, then Carrie is more successful.
Just choose what you consider successful and you have your winner.