Womanizer was a more impressive comeback. Britney had just comeback from a literal mental breakdown and the entire public hating her more than Osama Bin Laden. She had no platform like AI to hedge her bets, but the single still took off and went on to be one of the biggest selling singles of Britney's career.
On The Floor was a great song, loved by a lot of people, but Jen came back from...a pregnancy? And had the #1 show in America to fall back on and depend on for support. There was no way On The Floor was going to flop.