You know, what works for one doesn't necessarily work for another. I have had the soft side water bed and didn't like it at all so we gave that one away. My husband and I went through so many king-sized mattresses that when we bought our last one, he wanted two twin mattress that we could push together. This way, if I didn't like this mattress either, I could change my side and he could keep his side. Now, that worked just great and you might want to do that. This way, if you do make a choice and it still isn't comfortable for you, it won't cost you an arm and a leg to have a new bed and your boyfriend will be happy with his side.
We bought the two beds...pillow top with some firmness because I have a bad back. I need good support because soft beds and furniture hurt me more. The pillow top gave me the support but it wasn't soft enough so I bought a two inch Memory Foam bed topper and hated that, too! I couldn't roll over at all and was pulling my muscles every time I tried to get out of bed. (Some members love Memory Foam...we are all different.) Then, I bought a foam egg crate from Walmart and that helped a little but still wasn't the complete answer for me.
While on a trip, I slept on a feather bed mattress topper and it was heaven for me. I came home, headed to Bed, Bath, and Beyond and looked at their feather beds. They also had a fiber bed that felt the same as the feather bed but it could be washed so I bought that instead. Now I'm finally comfortable. I have my pillow top mattress with the foam egg crate on top of that and then my fiber bed on top of that. Now THAT gives me the support and the softness that I need. My husband passed on many years ago so I put the Memory Foam on his mattress for my dog. Even so, she and my cats always want to sleep on the fiber bed side! LOL
Buying the mattress this way will give you more choices and still will last a long time. These mattress of mine are about 20 years old now and still are just fine.
Remember, most likely you will still have more pain at night. That seems to go along with fibromyalgia. The more you sit and lay, you become more stiff and have more pain. I wake up about every 2-3 hours during the night, get up and walk to the bathroom. Boy, I'm in a lot of pain but by the time I head back to bed, I'm doing better. The next time I wake up, the same thing happens for me. I really don't look forward to going to bed for this reason. Once I'm in bed, I'm as comfortable as humanly possible for a fibromite, but I still have pain.
So, actually, all we can do is tell you what works for us. It might not work for you at all. If you buy a standard double or queen size bed you still could put things on your side of the mattress but it would spill over on his side. That's why I like what we did. Hope this helps you some.
Sherrine