Thermal mass, opinions?

I'm ordering a HFGH, 10 X 12. The greenhouses that I have had before were built, by me, from scratch with the north side heavily insulated. I'm too old for such a big project now..... I always used black 55 gallon barrels filled with water for thermal mass.

This new (to me) house has a lot of huge granite rocks that I could use instead. Since I have never used rocks, I'm wondering what are the pro's and cons of one over the other.

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Loudon County Pool and Spa Designer- Best of Houzz '12, '15 & '16