IMO, they are both wimps. Katy lost a wrestling match to a short, dumpy woman, but three cheers to Katy for even doing it. As for Lindsay, I've seen paparazzi video of her whining that Paris Hilton smacked her on the arm with a bottle. Obviously, LiLo did nothing about it. OTOH, there are internet reports that she challenged Jessica Simpson to step outside and throw down, and Jessica chickened out.
So . . . my guess is that Katy would win this battle of wimpy chicks. Katy has attitude, and she's taller and heavier. On the bright side, they both have world-class tits, so a nude match would be very arousing. Some T2TC2C action is called for here.