Hi Tempurpedic,
Both of these mattresses use high quality memory foam (5 lb or higher) but they don’t provide any details about the density of the base foam layers. More than anything though … both of them have a different design that will feel and perform differently so the choice would be more about which one is better for your specific needs and preferences in terms of PPP (Posture and alignment, Pressure relief, Personal preferences) than it would be about which one is “better or worse”.
The Tempur 8 lb memory foam is a little softer and more conforming than the Tempur 5.3 lb memory foam once it has softened with heat and would also be a little more durable. Quilting a foam layer and adding a layer as a pillow top are both methods that will change the surface feel of a mattress so the differences between them here would once again be a matter of preference.
In terms of durability … the quality and durability of the upper layers are more important than the base layers because a mattress will soften and break down from the top down. A difference of a few cm in the base foam is not as significant as any differences in their quality, firmness, and performance or differences in the memory foam layers.
Overall … both of these use good quality materials (for the memory foam layers at least because the base layers aren’t specified in terms of quality/density) and I would base your decision differences in how they feel and perform for your body type and sleeping positions in careful and objective testing.
I should also mention though that I personally wouldn’t consider either of these mattresses and there is likely to be much better quality/value available to you than any of the tempurpedic models (which along with most of the major brands are included in the mattresses I would tend to avoid). Post #1 here may also be helpful and the information, steps, and guidelines would apply to Australia as much as North America.
There are several “Australia” threads on the forum listed in post #2 here that also includes a link to many Australian manufacturers and some comments and feedback about some of them that may be helpful unless you really do have your heart set on a Tempurpedic regardless of their quality/value compared to other mattresses that use similar or better quality materials and sell for significantly less.
Phoenix