Sampras or Federer, who really is the greatest?

so today i started thinking about Sampras and the opponents he had during his career and the opponents Federer had in his career. Then I realized that Sampras had much tougher competition during his time (Andre Agi, Boris Becker, Michael Chang, Patrick Rafter, Jim Courier, Stefan Edberg) who are all remembered as some of the best players in the history of the sport especially Agi, Becker and Chang. Then you have Federer, and no disrespect to current players but none will really go down as amazing tennis players except for Nadal and Rod...maybe Murray in the future. So it makes me wonder, is his 15 grand slams really as great as Sampras' 14 considering the competition?