I've found a source on the internet and they seem to think that it is better than granite. Slate is great. I'm sure it's a matter of taste as much as anything else. I have no doubt that slate, as described on the site you provided, would be the equal to granite, or better - depending on your taste and how you intend to use the surface.
Slate is a superior surface in several critical ways.
Slate is fine-grained, relative to granite, and also much more uniform in composition. What this leads to is a material which is very durable, both to mechanical and chemical abuse. Both will chip if abused. Either will probably outlast you, so it may as well be the one you like.