Hi vonee,
In most cases … it’s the quality of the materials that are used inside most of the mainstream pillowtop mattresses that are the biggest issue … not so much the pillowtop design itself.
There are really two strategies here. One is having separate mattresses that are placed together side by side as a way to accommodate your different needs and preferences. The other is the strategy of buying a mattress and then adding a topper afterwards.
The first strategy is one of several that you can use that can accommodate couples that are in different weight ranges. There is more about this and some of the others in the first part of post #2 here.
The second strategy of adding a topper on top of a mattress can be a little risky because choosing a topper that is suitable for each of you in combination with a specific mattress can sometimes be almost as difficult as choosing a mattress in the first place (see post #2 here). Of course if you can test the mattress/topper combination in person with both of you on the mattress before you buy either one then the risk of making a “less than suitable choice” would be lower. If you choose a suitable combination the strategy can be a good one because as you mentioned you can replace a topper without replacing the complete mattress but the challenge and the risk involves making the most suitable choices for each of them.
There is more in post #13 here about the most important parts of the “value” of a mattress purchase but in terms of quality and durability … the only way to know this for any specific mattress is by knowing the type and quality of the materials inside it regardless of the name of the manufacturer or who made it (see this article). Many of the Amish made mattresses and foundations though do use good quality materials but knowing this for any mattress is always a matter of knowing the specifics about what is inside them.
Phoenix