According to my chart of Maximum Allowable Fiberstress in Bending Values they have a species selection of "Mixed Oak" giving all oaks the same value. Definately check it out further though.
Personally I prefer to mix species up. It is a wonderful way to more fully experience and understand trees. It also creates a nice color balance.
When I lived in the East I used up to 10 species in a single frame. Now I live in the west and have less variety. I say go for the mix not because of price (although that is nice) but for the personal satisfaction of creating a Timber Frame that more fully represents the forest it came from.
-Collin
