KISS Mattress Foundation

I’m about to pull the trigger on a KISS Mattress for my mom. At least I think I am. Alas, I got to the foundation, which she needs for her bedroom setup, and then I see KISS has made my choice extremely difficult by offering two different foundations. How do I choose? Should I ask her to measure her current foundation and use this as the deciding factor? One is 5 inches and the other is 8. We’re only talking about 3 inches, right? Decisions, Decisions. If anyone has insight, please let me know. I’m at a distance from mom so I need to give her very specific directions if she has to measure depth.