Hingis is by far the better player. Late in 96 when she was nowhere near her prime, in fact she was 15 most of the matches, she was still such a hard opponent for Graf in her dominant prime. Beating Graf on clay in 3 sets in Rome. Then having 5 set points on Graf in the 1st set of the U.S Open semis, if she had taken one she probably wins the match and the U.S Open that year. Then taking Graf to 5 sets in the year end womens event final, a match she would have probably won in 5 sets without cramps since she was rolling in the 4th set up 5-1 with all kinds of momentum when she started to cramp. Only in the Wimbledon 4th round did Graf win relatively easily.
Graf in 97 was scared to be beaten by Hingis so her injuries mysteriously got worse, and she kept losing to avoid getting beaten by Hingis. If Hingis was 16-26 while Graf was 17-27, and faced the competition Graf had she would have atleast 30 slams and a record number of U.S Opens, Wimbledons, and French Opens. Every single slam Graf won Hingis also would have won, there isnt a single one Graf won you can say Hingis likely would not have won vs the same opposition, but she probably wins more too. I dont see Hingis losing a U.S Open final to Sabatini or Sanchez Vicario had she been in her prime then instead of now. I dont see Hingis letting Sanchez Vicario win either of her first 2 French Opens had she been in her prime then. I dont see Hingis letting Garrison and McNeil stopping her from winning a couple of Wimbledon titles, that went to 33 year old Navratilova and Conchita Martinez because of Garrison and McNeil. Hingis may have also been better able to snag a few more slams in 86-87 with an aging Navratilova and Evert then the mere 1 Graf managed. Hingis dominated post-stabbing Seles so thoroughly she could have managed to win a couple of the non-Wimbledon slams in 91-92 all which went to Seles and none to Graf. Had Hingis managed to be healthy enough to play the 95 and 96 Australian Opens, which Graf wasnt she may have won 1 or both of those. Once she hit her prime she dominated post-stabbing Seles which 96 Australian Open Seles was, and she dominated Pierce throughout their careers.
Unfortunately for Hingis she got stuck in the era of the most brute power in womens tennis in history with the Williams and Davenport. These players overpowered her and dispirited her, and kept her from winning the 30+ slams she would be approaching by now had she been born 11 years sooner.