we intend to buy a new mattress

we currently have a slatted bed base with Layezee 'over 1000' pocket spring mattress. it is about 11 years old.
My wife is 9 stone and I am 11.5 stone.
we currently find this mattress too firm. So we have been looking at medium mattresses. Our budget is under 700.
We have been looking at a Hypnos bedstead one- it is a king size mattress.
what would you recommend