Hi Krasen,
Just in case you haven’t read it yet … the first place I would start is the tutorial post here which has all the basic information, steps, and guidelines that can help you choose a mattress that is the best match for your unique needs and preferences.
If you let me know your city or zip code I’d be happy to let you know of any of the better local options or possibilities I’m aware of.
If you do decide to make an online purchase and you are committed to memory foam (which it appears is your preference since all the mattresses you mentioned are memory foam mattresses) then post #12 here includes some of the better online sources of memory foam mattresses I’m aware of in a wide range of budgets.
Many of them compare their mattresses in terms of “feel”, comfort/pressure relief and support/alignment to some of the more well known brands. This is most often Tempurpedic but sometimes some of the other larger manufacturers such as the Serta iComfort line or the Sealy Optimum line as well so you can test a local mattress that is a good approximation of one their mattresses to make sure it is suitable for you in terms of PPP. Some good local testing in addition to a more in depth conversation on the phone are important parts of the success of an online purchase.
I would first narrow down which of the three mattresses you mentioned is the best match for you using the testing guidelines in the tutorial post. This way you will have a single reference point as a target instead of many. If you choose one of the Tempurpedics then you would have more options to choose from (because more of the online manufacturers and retailers use tempurpedic as a reference point than any of the other commonly available memory foam mattresses) but some manufacturers also use other mattresses as well (such as the Brooklyn Bedding iPedic Saviour which is an approximation of the iComfort Savant).
If you only need a mattress to last you 5 years then 4 or 5 lb memory foam would be fine in terms of durability and you don’t need to focus on the highest density memory foam in the 7 or 8 lb range.
I would also make sure though that you don’t choose a mattress that is on the edge of having comfort layers that are too thick or soft for good alignment because if you do then even a little bit of foam softening can put you over the edge in terms of support and even if the mattress isn’t worn out and may still be fine for someone else with a different body type, it may not continue to provide the support you need to keep you in good alignment (see post #2 here). All mattresses will soften over time so I would make sure that if you have choices that are approximately equal that you choose the one that is just a little firmer rather than a little softer so it still has some “room” to remain suitable in terms of PPP after it softens a little as this can also make a difference in the useful life of a mattress.
Phoenix