You don't see why we are comparing their success against Sampras? Obviously because he was the best in the 90s on the faster surfaces. Only true way to really link the two players in terms of success as that is the only common element. Even then it is still extremely subjective, but it is much less subjective than trying to point out the field of players, surface speeds, etc.
Courier had his chances in 1991-1993 as he was in his prime while Sampras was still clearly not in his prime, so you can make the case either way. Both Hewitt and Courier faced a non-prime Sampras in their times and both managed to win 1 US Open around the same time with Sampras entering his prime in 1993 and leaving his prime in 2000.
By 1993 Sampras had already 3 GS titles and 1 final, as well as 1 year end championship and 1 Grand Slam Cup. So he was clearly at the beginning of his prime, and certainly more competitive and hungry than he was in 2000, when Hewitt started beating him.
Also, the Sampras-factor clearly favors Hewitt, because Sampras was the worst possible mactch-up for COurier. It was Sampras who originally exposed Courier´s game, exposing his backhand by first attacking his forehand-side. Sampras was the first to figure it out, and that´s pretty much where Courier´s downhill started.
Hewitt, on the other hand, was more than happy to keep passing Sampras, who was getting older and slower by the beginning of the 2000´s.
So it´s not really fair to compare Courier and Hewitt by comparing their results against Sampras.
By the way, Courier never won the US Open.