bunk bed mattresses

HELP. I'm over-thinking this! I'm torn as to which mattresses to get for my girls' new bunk beds. The manufacturer said I could get 6 or 8 inch thickness and it will still leave plenty of guardrail on the top bunk. I've narrowed it down to a 6 inch from room and board or an 8 inch at bedroom source (both are organic, chemical free etc, which was important to me).

What size mattresses do you have on your bunks? Any advice is welcome! I am truly obsessing over 2 inches!