This is an old post, but here are my thoughts. One, I don't like grill pans that have high ridges. It keeps the food off of the main pan and it doesn't sear as well. So the ridges should be modest.

Second, theoretically, a thick aluminum would be better, it's lighter and absorbs the heat faster and spreads the heat better. But, aluminum almost always has a non-stick surface- this is O.K. for a smooth lower heat griddle, but not as good for a higher heated grill.

Ideally, I would like aluminum clad with a thin top of carbon steel, but it's not made.

So, cast iron ends up looking pretty good for a grill.