I understand the question, but I'm going to look at this a little different since I think the choice likely comes down to "what tool for what job". If we're talking about which one feels better in my hand, and which do I think is more worthy of praise for how artistically it is put together: I'd go with steel all day long. For example, I love a Stainless 1911. They are absolutly beautiful, and unbeleivably well balanced when shooting. A real joy.
But my view on this is more of a "if I could only have one, what would it be?" Now I'm thinking about needing to carry it, weight, comfort, capacity, and will it work if I'm not able to clean it properly and regularly? Will it hold up to abuse? These sorts of things.
In that case, I'd say a Glock 23 (so, plastic). That's where my vote went.