Tire compound has much more to do with rolling resistance in my experience. The tougher the rubber the less it deforms. For example, in 26inches, I found the Marathon plus at 1.75 and 70psi ran much faster than an Armadillo 1.5 at 90psi. The rubber is just better.

In regards to 26 vs 700c. I think the weight of the rim and the bikes geometry have a much bigger impact that the wheel size.

These are just all personal opinions based on what little experience I have.