All wool mattress Devonshire Vi SPring?

Hello
Hope you can assist. We are pondering whether to purchase an all wool mattress or mix e.g. cotton horsetail/hair, cashmere, wool - I have heard that all wool can flatten down over time and lose some of its support? Also if it were to be a all wool mattress then should we go for medium rather than soft - we are both light in weight 10st and 11.5 respectively.
thank you for your help.
Mrs Saunders