Pocket Sprung with Topper or Memory Foam

My wife and I have decided we would like a new memory foam mattress (king sized to go on sprung slats with 1 inch gaps. I'm 11st, she's 10st). We looked at a Tempur Cloud 21 (perhaps it was a 22) and both really liked the softness of the top layer, such that it met our requirements. I'm determined to get better quality for a lower price, hence I'd prefer to purchase a Hybrid 4 mattress. On your product page, you describe the Hybrid 4 as being like the Tempur Cloud 27, which we tried but found slightly firmer than we wanted. How soft is the top layer on the Hybrid 4? Is it noticeably 'sink into' or more 'rest on top of'?
The other thought would be one of your foam toppers on an Origins 1500 mattress. The trouble was, we tried several untopped pocket sprung mattress (including a 10k Vispring one) and really didn't get on with them, so I don't know how suitable a pocket sprung mattress with memory foam topper would be.
I'd appreciate your opinions. We're currently on a £200 ikea foam mattress that feels too firm to both of us, so the comfort of the top layer is really important. I have aching hips and legs that have persisted through the last two mattresses we've had, so I'd like something that alleviates the discomfort.