I found a Danish memory foam mattress called "Celsius." Unlike the Temperpedic, you can put the Celsius directly onto platform bed slats rather than use a foundation. The Celsius is also about half the price of Temperpedic. But I am wondering which is the preferable mattress.

I haven't heard of this company before, but I do know that you can also put a Tempur-Pedic onto a good quality slat foundation, really shouldn't matter.   As far as which mattress is preferable is a whole other can of worms.