itsreallyconc - You're exactly right with the thermal mass, 4" would be better than 2", but that's a substantial dead load on the floor (I'll leave the thermal discussions for another day).
My first post identified looking to get the concrete polished and leave that as the finished surface, however, the prices for polishing alone (placing concrete is additional), was $8-10$ / sq. ft.. We have no problem tiling ourselves for under $4/sq. ft., which is why I am posting the question to the forum about the substrate. I'm still leaning towards a portland cement mix with small aggregate. Thanks for your reply.