Copics and Stampin' Up! markers are just different. It's just what one prefers. The SU! markers are water-based. Copics are alcohol based. With Copics you can colour over the same spot and the paper won't start to be destroyed. With SU! markers you can ink up rubber stamps...
$8 per marker is outrageous. Oozak.com has them for $4.99 per Copic Sketch marker (if you log in), and free shipping over $50.
Edited to add:
Oh, to answer your question about what I can do with them that I can't with SU! markers is, I can uh, colour in the same spot over and over... Kind of a bad habit, but... well, when using water-based markers, I do have to be aware of not flooding the paper with too much ink (water) and warping it or destroying it. I only have a limited number of swipes. With Copics, I feel more relaxed, like I can swipe without limit.