Hi Sonja,
As you know, Mattresses.net is a member here and I think highly of the quality and value of their mattresses and of course their knowledge and experience.
It would probably depend on whether your platform bed has a solid surface or slats (and if it has slats on whether they flex or are rigid and on the distance between them). You can read a little more about the potential risks involved in a solid surface base in post #7 here and the post it links to.
Both a slatted foundation and the bed rug would work well for providing ventilation under a mattress although they would raise the sleeping surface of the mattress to different degrees and this may also be a factor that is important to you.
Phoenix