David, here's what I can tell you:
They are BOTH excellent setups. They are BOTH overpriced! That said, everyone I know who has dropped the $$ for these high-end mattresses has sworn up and down it was the best investment they have ever made! (assuming they were NOT avoiding admitting making a mistake)
My Dad and stepmom have a Select-Comfort. They LOVE it. My stepmom is like the princess and the pea, and has horrible back problems. The SC mattress gave her rest and mobility again. She says that she can actually sleep 2 hours less and feel MORE rested on the SC mattress.
SC will refund your money if you don't like it, but you have to box up and repack this huge setup (good luck) and they won't pay return shipping AFAIK.
The Tempur-pedic mattress is my personal favorite, though. It is unbelievably comfortable. I went to a store once and sprawled on it for a little bit. My wife ended up having to wake me up!! Yes, I fell asleep in like 3 minutes. That's never happened to me before. I have no other experience with a TP mattress, but I have used their pillows for quite some time and I can't sleep without them now!
The drawbacks to the TP mattress are more than just cost. First, it has a lot of give to it, so your bedroom olympics with the wife may be a little more difficult. Second, it will get as hard a concrete if you let it get too cold in the room (50°). I own TP pillows, and you can seriously injure someone with them if they get very cold. So you won't want this kind of setup for camping.
Personally, I think I would try a tempur type of mattress topper before I went hole-hog on the mattress. Truth is, only the top 2 inches or so of memory foam really matters- as long as you have support under that layer, it will be comfortable.
HOHN