If a student's rank or a student's mark in a particular course is the only piece of knowledge that you have about them, the rank is far more valuable. There is no way to evaluate the performance of a student who has a raw assessment mark of, say, 75. However, if instead of that mark you knew that the student was ranked 2/35, you can say with a reasonable amount of certainty that the student is doing well.
As students rarely know the marks of every other student in their class, they are forced to guage their performance by their rank. A student's mark is only significant in the context of all of the marks for their entire class.