You're going to learn different things in different places. Small companies will give you a lot more breadth of knowledge because you won't be able to specialize in any one area, but will need to be as good as you can be in lots of different areas. Large companies will teach you a lot more about process, documentation, and scale. Plus, with a large firm, you'll be able to specialize and drill down on a particular skill set. Then, of course, there exceptions in both directions. I worked for a small organization that had a larger database than any I manage today.

I'd just be careful about any work environment that is labelled "challenging." It usually means it's just messed up, hard to get anything done and stuff is breaking constantly. That's not a challenge you want.