What type of reference map is best to use to distinguish plains from grassland?

I'd say agricultural productivity. Guess that's close to land use, is it not?
 
Civ IV bases it on latitude, with jungle being on the equator, going north or south getting you desert, then plains, then grassland, then tundra, and finally arctic at the "poles."

Thus, I'd say it's reasonable to assume that plains are supposed to represent an area more arid and/or typically warmer than grasslands, but not to such an extent so as to be rendered barren, like a desert is.

I'll have to suggest you go with a rainfall map, and a map displaying average temperatures would probably be good to look off of too. A land use map is something I would use for placing resources.
 
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