If I were you, I'd bound the uncertainty by running two quick simulations. In the two quick simulations (course mesh, low tolerences, etc.) test the difference in results between the two specific heats. Depending on the results which you choose to measure, the difference between the two simulation may not be significant. I say this because the difference between the two specific heats listed above is on the order of 3%.
Further more, considering that you're working with rock (which is an organic material) you'll find that there is a considerable amount of variation between rock samples.
With all of this said, I believe the software will calculate the combination for you because you enter in the pour properties as well as the fluid. Thus, you'll probably just want to put the rock properties in.
All the best,
Nick LuysterOnline SolidWorks Simulation Training
Can you elaborate on what your goal is and what properties you have for your porous medium and which you don't? The reason I ask is that for a porous medium with heat transfer, the inputs aren't as straight forward as you think. We just worked on a project for a customer trying to come up with a method for heat transfer, including speaking with the developers and it is relatively involved.