Help choosing a mattress for side sleeping, lower back pain

I am trying to choose a mattress (with base) and am getting totally confused by the various options out there. I am 5'11", 11.8 stone. I am a slide sleeper and have occasional lower back pain related to disk issues. What would you recommend? What should I be looking for in a mattress?
My current mattress is a Sleepeeze Concept Backcare 1000 which has been very good when sleeping on my back, but I sometimes think is too firm when sleeping on my side (back doesn't feel straight). Current bed base is slatted bed frame, which I think is not ideal (feels like the mattress sags).