Picking a mattress thickness/height is a matter of preference and also depends on the height of your current bed setup. All Saatva mattresses provide adequate support regardless of mattress thickness.
If you prefer a taller bed, consider placing your mattress on our taller 8.75" foundation and add a metal frame that sits 7.5" off the ground. If you prefer a lower bed height, consider a low-profile platform bed frame which does not require a foundation or box spring.