We tried the Sleep Number bed but my dh couldn't tolerate it (don't ask me why; I liked it!).
If you want a mattress that doesn't sink in the middle, avoid pillow tops. The pillow tops are notorious for that.
The best mattress and box spring set we ever purchased was a Stearns & Foster brand, near their top-of-the-line but not quite (the t-o-l was a pillowtop). All of the mattress companies change their mattress names periodically, even if they don't actually change the mattress. The one I liked had one side for winter and one for summer -- different fabrics -- and no pillowtop.
As others have mentioned, the box spring can make a huge difference.
One thing I will never do again is buy an overly thick mattress (current one is a pillowtop; dh insisted......). *Everything* becomes more expensive -- mattress cover, sheets -- because they all have to have the extra thick sides. And that type of sheet set rarely goes on sale and, when it does, isn't in any color I want.