Ya gotta get past the idea that one particular characteristic makes one head better than another. It's the whole enchilada that matters.

Generally, you can run higher compression with aluminum heads - around 10 - 10.5 to 1.

Aluminum heads are significantly lighter, which is a good thing.

Being made of aluminum doesn't necessarily make it a better head. For instance, there are Brodix and World iron heads (for just a couple of examples) that are much better than stock Chevy aluminum heads.

Given equal flow and compression, though, I would almost always go with aluminum.