Lois,
I order from Mpix sometimes too. The metallic really isn't that much more than the regular in larger prints. If you get the FedEx next day you can know right away.
Personally, I think there are too many greys and muted tones to make the difference worth getting metallic paper. I find that metallic works best with bright colors. Grey tones don't work as well. Stark whites or black yes...greys...not so much.
If you convert to b&w the true b&w paper would look good.
Just a thought.
Lori