bed base for Artisan 004

Hello,Keen om the artisan 004 mattress and I like the comparison to vispring matresses. Our existing bed is a wooden slat base so which firmness should we go for if its a medium firmness using your scale (and i presume for divans).I am 5'11 and 14 stone and my wife would also come under the medium rating for your mattresses.Let me know what you think.Kind regards,Brendan