I think Lady Gaga has more vocal ability but she wouldn't be where she is today if Madonna hadn't broken through in the 80's. Also Madonna's songs have lasted for and are still popular after three decades.
Style? Madonna. Gaga owes too much to Bowie/Alice Cooper, etc. Musical ability? Gaga by a COUNTRY MILE. Madonna is only OK. Listen to Ray Of Light; her voice gets really thin as it goes higher. Compare that to the bridge of Bad Romance. No contest.
