I have a Tempurpedic and have had it for about 8 years.
If you are at all concerned about chemical exposure or allergies, I would consider a mattress that is 100% natural latex as opposed to the 2 brands you are considering which are synthetic.
Do a little research before you buy.
I bought mine from these guys but there are many others.
^-- True. I didn't really sleep on my Tempur for the first year, I was away travelling and the noxious stuff pretty much escaped by the time I was regularly sleeping in it. It was pretty strong for the first week. So many household items are bad this way, new carpet is particularly terrible in general.
There are certain things that the memory foam beds are not good for.... like running uphill on soft sand.
In addition to those things, if you are someone that turns in your sleep, you may or may not start waking up every time you flip- or not. Your partner won't wake you up when they flop though.
^-- Also true. Great for sleeping, bad for sex. Well, actually it's good for sex if you really like a workout. Think of being mired in a bog. It can be fun if you're athletic.
I like the bed because I hate being jostled by someone else moving around, you can jump on it next to someone's head and they won't wake up. I, in fact, did so.
I also don't like pillowtops very much. My back enjoys the Tempur foam.
They do tend to get hot if you're someone who puts off a lot of BTUs. I am usually chilly so it works out great for me. My bed is older material, not the newer type, so when it is cold the bed gets very hard. I solved this by having an electric blanket (oohhh how I love that electric blanket) and warm up the bed a little bit while I take my nightly shower if it's cold.*
However, had a natural latex bed been available at the time, I would have looked into it seriously. It may be my next bed, though my current hasn't shown any sign of wear yet.
*I don't have central heating and sometimes it's 40 degrees inside my bedroom (windows don't seal, single pane, SoCal does get chilly) and thus the electric blanket with my wonderful squishy bed = ahhhh.