Artisan 1500 soft or medium

Hi, an equiry on behalf of my mother who is set on your artisan 1500 but doesn't like the sound of 'soft' as she says her old mattress is soft and that's why she's having a new one :) She is 5'7 and weighs 9 stone. Your table has her on the border of soft and medium but I noticed the upholstery of the artisan 1500 is on the firmer side. Does this mean a soft mattress would feel a bit firmer so it might be better to go for the soft version? Or if she says she prefers a firmer mattress should she just go medium? Thanks in advance for any help. John Lee