See
these recent posts from the grand list of simple questions for a more detailed comparison. The TL;DR is that a Pickaxe is practically always better for mining efficiency, but the hardness of rocks you can mine depends on quality. When I crafted my ~q15 pickaxe, it was much better at the nodes it could mine than my ~q70 stone axe, but I was left unable to mine my main ore node with it (which I could mine with my ~q70 stone axe).
Always go for a pickaxe unless the node is hard and there's a big difference in quality.