It depends on if your ranking system branches more or roots more. Branching would mean many different "classes" but only a few levels of hierarchy ("ranks"). Rooting would be only a few "classes" but that have many levels of progression (more hierarchy).
In the case of branching it would likely be easy to come up with three different ranked names for each class. (Mage, Master Wizard, Grand Sorcerer).
However if you only have a few classes but each has 100 possible ranks, good luck coming up with 100 logical hierarchical titles, so simply giving Level #'s would make more sense.
EDIT: Ideally you could do what Xabre states in the comments and do both. That way you can have all 100 levels of progression numbered and then have a rank title that changes every 30 levels or so (however many names you can come up with), to keep it interesting.