Hi Nada
For a good mattress to last, I think it should have a minimum of 1000 springs and those springs should be pocket springs not open coil springs. Look for descriptions that say that the sides of the mattress are reinforced so that when you are sitting on the edge of the bed, it doesn't get dented over time.
If your dh has a bad back, it would be best if the mattress was on a divan base rather than a trendy bedstead (yes, I know the French looking beds are very alluring!). A slatted base is not a good foundation for a mattress as it will eventually sag and not give his back enough support.
I don't know much about orthopaedic beds, I'm not convinced by all these memory foam fillings and mattresses but perhaps I'm just not with it.
If money isn't a problem, you can't go wrong with a Vi-Spring. Many hotels use this brand and for good reason, they give a very comfortable sleep, my teenage bed had a Vi-Spring mattress and I still enjoy returning and sleeping on it, your back just feels very supported. They offer the option of having two types of support on one bed (divided down the middle). My parents have this type and tell me (way too much information!) that it is very comfortable and you can't feel the line of division. I think that the brand also gained the most points in Which magazine's latest bed testing.
Couldn't tell you much about other brands, Sealy is probably quite a reputable one, they've been going for years however I am not overly impressed with Milbrook, just personal opinion, could be good for someone else.
If I had enough space, I would opt for a super king size! More room to spread out and the bed gets less hot. I'd better start saving. HTH