Hi lnr103,
The first place to start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices. If you follow the steps one by one your odds of making a very suitable and durable choice will be very high.
The first step outside of gathering some initial information would be to identify the better retailers or manufacturers in your area and do some mattress testing to see which type of materials or types of mattresses you prefer so you can narrow down your options.
Any type of mattress can make a great choice as long as they are a good match for you in terms of PPP and you know the specifics of all the layers in the mattress so you can confirm it uses good quality and durable materials (in the comfort layers especially which are usually the first to soften and break down).
The weak link of a mattress (including innersprings) is almost always in the upper layers of the mattress, not in the support layers.
If you are looking at and comfortable with polyfoam or memory foam then I would make sure it is either North American made or certified by CertiPur for harmful substances or VOC’s. While some people may still be sensitive to some materials that have the CertiPur certification (there will be some VOC’s in any material) or for personal reasons wish to avoid any synthetic materials completely … for most people they would be “safe enough”.
Oeko-Tex is a little higher level and more stringent certification for harmful substances and is the most common certification for latex (synthetic, natural, and blended) although there are a very few memory foam or polyfoam materials that are also Oeko-Tex certified but they are mostly European.
For those who for personal reasons want to take on the more difficult challenge of researching all the complex and often misleading issues surrounding mattress safety in much finer detail or who may have a different answer to “how safe is safe enough for me?” than most people, then post #2 here and the other posts and sources of information it links to would be a good place to start.
I agree with you about Essentia and you can read more about my thoughts and discussion with them in this thread and this thread and posts #3 and #4 here. The information and claims on their website are misleading IMO and they are a manufacturer that I would be very cautious about considering.
Out of the list you mentioned I would tend to avoid Tempurpedic, Simmons, Bedinabox (see post #2 here), or any mattress no matter who manufactures it where you aren’t able to find out the specific quality of the materials that are inside it or where the quality/density of the materials are too low.
If you let me know your city or zip code I’d be happy to let you know of any of the better options I’m aware of in your area.
Phoenix