Hi shelilah,
What city do you live in?
It’s unlikely that you are right but it would depend on the specific densities of the foam layers inside the two mattresses you are comparing.
If you have spent more than a couple of hours reading some of the information on this site it’s also very likely that you know more about mattresses and the materials inside them than most of the salespeople in the mainstream industry.
If you can’t find out the thickness and density of all the foam layers in the mattress then I would tend to use the mattress + topper strategy and choose a firmer mattress that minimizes the amount of “unknown” foam because they would almost certainly be low quality and less durable materials.
Any stain on a mattress (even a watermark) can void a mattress warranty.
There is more about the pros and cons of different types of mattress protectors and some examples of each of them in post #89 here.
Both of the mattress protectors you linked are similar and both have a polyester fabric with a waterproof and semi breathable membrane. Depending on the mattress and on all the other variables that can affect sleeping temperature … some people do notice that they sleep warmer with these types of mattress protectors because the airflow through the membrane is more restricted.
As you mentioned some mattress protectors offer a warranty if they are purchased at the same time as the mattress (as part of the same purchase) but these can be very difficult to make claims against if you have an accident that soils the mattress because they will often claim that you didn’t use proper care instructions and deny coverage. In some if not most cases they will send you a cleaning kit so you can remove the stain yourself but for them to actually agree to replace your mattress would be very rare (almost unheard of).