I personally find sleeping on foam of any kind impossible - my body even in sleep produces a fair amount of heat and I have yet to find a foam mattress that doesn't result in the side against the mattress overheating. But that is really a personal issue and unless you have had the same experience not a normal problem. So ... the choice is really what feels best for you. But I would go with high quality - high coil counts, and heavy duty wire if you go inner spring - you will not notice it in the first year, but cheap stuff will start to develop lumps, change in comfort, etc.
I have a lot of success using mail order company US-Mattress.com ordering 5 mattresses over the years from them. They have a fairly good softness rating system, a huge selection, and a good replacement program should the mattress not be to your liking once you try it.
I have also found that mattress companies (like many electronics companies) create 'model identifiers' that are unique to each store (at least at the discount stores) so that the price guarantees that are offer are impossible to use for comparison pricing. The same model will be called 'Daytona' at one store and 'Riviera' at another for example.
Finally - the comfort level experience - lying down on a mattress in store for five minutes to test it is very different from actually sleeping eight hours on it at home. You tend to buy a harder mattress on the 5 minute test drive than you want for a 8 hour sleep. It is why sitting on the floor for a few minutes to play with your kids is OK, but actually sitting on the floor to watch a 2 hour movie becomes really uncomfortable and you want at least a cushion.
And while we tend to think we only need to pamper ourselves as we get older - good rest at any age is important, so I would get good mattresses for anyone at any age.
Another thing - box springs really no longer serve the 'spring' function they used to - that is all now contained in the mattress and the box spring just provides a flat solid surface. And with mattresses becoming ridiculously thick, unless you like 'climbing' into bed - go with the lowest profile box spring you can find. I find I have had to remove the wheels on bed frames just to get the bed to a normal height.
King/Cal King/Queen is all personal choice and bedroom space - take measurements in your bedroom and lay out where the bed would come to if you are upgrading in size. And check your access to the bedroom especially if it is available only through a stairway - mattress are no longer as bendable as they used to be and with the added thickness even on a diagonal they may be hard or impossible to get from the front door to the bedroom - and king size box springs are actually two piece, but queen size are one piece. Cal King is really only important if you are well over 6'