Hello Ben,
Well, I'm no expert but all I can do is tell you what I was told this week when I went to another session of the Back Rehabilitation Programme classes - I was sent to them by a physiotherapist following my own back injury. The classes are run by physiotherapists too. They basically said that a lot of money has been needlessly spent by desperate people on expensive orthopaedic beds. They said Orthopaedic = Rock Hard which can be TOO hard. They said to always have a solid firm bed BASE and the mattress was a matter of choice - whatever type of mattress you feel is comfortable for YOU but added medium to firm are usually the best. I bought a Slumberland firm bed recently which has helped a lot once I got used to the contrast of sleeping in a saggy old mattress prior to that. Had a couple of sleepless nights initially till I got used to it. I sleep like a log now usually. The physios said that it's a matter of choice so long as you're comfortable but generally speaking an older person who has less suppleness/give in their joints may be better with a slightly softer mattress.
Can't really comment about the memory mattress type - the physios didn't mention them specifically. I DID lie on one briefly in the showroom but they were out of my budget at the moment. I'm guessing they would be okay providing you lie correctly (ie posture-wise) if you see what I mean (a bit like not slouching in a chair for example)
The biggest message from my class this week is that you shouldn't waste money on expensive Orthopaedic beds as they are TOO hard.
Good luck - hope you make the right choice and that this has helped a bit. As I say I'm not an expert.
Crystal Spirit x