My personal headcanon is that while both Seers and Mages deal with knowledge, the difference is in how they obtain their knowledge; with Seers obtaining their knowledge through outside means (crystal orbs, tomes, walkthroughs, etc.) whereas Mages primarily obtain their knowledge through experience and action. It's kind of like how in DnD, wizards have to study magic to learn to use it, while sorcerers have innate powers that they need to hone.

Consider Rose contemplating the cue ball vs. Sollux's unparalleled expertise when it comes to coding. Consider Terezi using her coin flips to make decisions vs. Meulin's natural obsession with relationships.