My wife and I are in the process of purchasing a king size ecomfort matress. We have friends who love the matress...one of them had a tempurpedic for years and likes his new ecomfort matress better. He reports that is sleeps much cooler than the tempurpedic in the summer time.
In regards to the difference between inner spring mattresses and the ecomfort, most new matresses have some form of memory foam as their top layer, even if they do not have a pillow top. It is very difficult to find a quality matress without a memory foam top layer. Everything we have found (and my wife has done a ton of research) points to the ecomfort sleeping cool. We also looked at similar products that are eco-friendly and ecomfort seems to have the most reasonable deals.
Hope that helps. If you have already purchased a matress let me know what you think of it.