There's more to your choice of terminals than just the lounge. IMHO the RCC at both are about the same, although I hear a lot of complaints about the IAD matrons.

That being said, IAD is always a zoo to negotiate. Too crowded for the narrow hallways. The mobile transit lounges are a pain.

On the other hand, coming inbound through ORD, you'll clear immigration and customs in one terminal, and then take a train to another terminal. A real pain. At least in IAD, you can negotiate both at the midfield terminal. I guess it's this latter that would have me favor IAD, eventhough I despise it's crowded conditions.