Hi Minnesota,
If the mattress looks something like this (like a separate pillow with recessed edges sewed to the mattress) then it’s a pillow top. If the mattress has flush edges like this with a “box” on top … then it’s a Eurotop or a box top. Post #2 here has more about the differences between them. They are different designs with different combinations of layers. The “best” choice is the one that your careful testing indicates is the best match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). If two mattresses are very close and it’s difficult to decide between them I would choose the one that is a little firmer rather than a little softer because it’s easier to “fix” a mattress that is too firm (with a topper) than a mattress that is too soft.
A topper can be a suitable choice for a mattress that is too firm and needs some additional comfort and pressure relief but it’s not a good way to “fix” for a mattress that has developed soft spots or impressions (see post #4 here).
Whether the 1.5 lb polyfoam was a “weak link” in the mattress would depend on how thick it was (see the guidelines here)
That’s a very general question so the best guidance I would have would be to follow the steps and information in the tutorial post but two of the more important links that it includes are post #2 here which has more about the different ways to choose a suitable mattress and how to minimize the risks involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase that can help you make more meaningful comparisons between mattresses.
Phoenix