Firstly, Comfor-pedic mattresses use a different type of memory foam (NxG or Next Generation) to that used by Tempur-pedic that combines latex and visco. Simmonds claims this disperses heat much quicker - a bonus in hot climates where overheating and sweating can disturb sleep.
Tempur-pedic claims their proprietary Tempur® foam is temperature sensitive, allowing it to mold better to your body shape, relieving pressure on sensitive areas.
The reason for this heat problem may lie in the differing density of the memory foam used in each brand. Tempur® foam is typically around 5.4lb per cubic foot, providing excellent support and weight distribution, so you are unlikely to disturb your partner if you move around, whilst that used in Comfor-pedic mattresses varies quite substantially - between 3.5 - 5.5lb per cubic foot.
The problem is that the higher the foam density, the more likely it is to cause heat build-up. But this means the quality is better, so it is a bit of a Catch-22. (See: www.myfoammattress.net/guide/complaint.html) Some sites also claim that different climates can the hardness or softness of memory foam. (See, for example: www.best-mattress-reviews.com/tempurpedic-mattress.html#ixzz)
Comfor-pedic memory foam apparently bounces back into shape much quicker, making it easier to roll over in bed at night. However, Tempur® foam may hold the shape of your body for longer, resulting in less need to roll around to get comfortable.
This could be because Simmonds Comfor-pedic mattresses use a thinner layer of foam (typically between 3.5 - 4.5 inches, depending on the model) compared to that of Tempur-pedic (usually 4.8 inches) making the Comfor-pedic feel firmer. However, if you prefer a softer mattress without compromising on back support, then the Tempur-pedic may be a better choice.
A breakdown of the different models and their statistics can be found at www.reviews-mattress.com/comforpedic-vs-tempurpedic.html, although there is no way of knowing how up-to-date this list is.
Warranty lengths also differ. Tempur-pedic offers a 20-year warranty whilst Simmonds provide a 25-year one on Comfor-pedic mattresses.
In the end though, buying a mattress comes down to personal preference, so it is best to try out both before you buy.