Simba mattress unsuitable. What mattress might suit us both

Our previous mattress was a 14 year old pocket sprung Sealy. Thinking it was time we had a new one, I succumbed to the hype and rave reviews and ordered a Simba. Four weeks in to the 100 night trial and my husband loves it, finding it much more comfortable than the old one. He periodically suffers from disc-related lower back pain, though it's okay at the moment (which may or may not be a coincidence). I, on the other hand, am finding the Simba much too hard and much too hot. Because of a pre-existing condition, I always sleep on my left side. I've had to start using an extra pillow to stop my neck and shoulder hurting and I ache pretty much all over when I first get up in the morning. I also find my entire left arm "goes to sleep" and I get pins and needles. Worst of all, I'm waking up 2 or 3 times a night, for varying lengths of time, because I'm just too hot! We're using cotton sheets and a 4.5 tog duvet. I've convinced myself that if we go back to a pocket sprung mattress all my problems will be solved. I'm 5'2" and weigh approx 10 stone. My husband is 5'10" and weighs approx 12 stone (and sleeps both on his side and back). I went to have a look in Carpetright today, our only local bed store, and took details of the Rest Assured Kensington and Sleepeezee Gladstone. However, having come across your website by accident this evening when searching for reviews, I'm wondering about the quality of these mattresses. Please can you suggest a type of mattress, and level of firmness, that could suit us both? Ideally, I don't really want to spend more than the cost of the Simba, but we could go a bit higher if necessary. Many thanks for any advice.