Hi Terry,
I would keep in mind that a mattress warranty only covers defects and not the loss of comfort and support which is the reason that most people will need to replace a mattress. Warranties have very little to do with the useful life of a mattress and how long it will last you (see post #174 here). Knowing the quality of all the materials inside any mattress you are considering so you can identify any weak links and make meaningful comparisons to other mattresses is always much more important and a more reliable way to assess the durability of a mattress than the length of the warranty .
A trial period is just in case you make a mistake in choosing a mattress that is a good “match” for you in terms of PPP and has little to do with issues that will develop with the mattress itself. Exchange and return policies are built into the cost of a mattress so the people who don’t return a mattress end up paying for the ones who do (see post #3 here). Careful and objective testing can significantly lower the chance that you will choose a mattress that isn’t suitable for you but if you are still uncertain about whether a mattress is suitable for you even if you have tested it carefully then a good return or exchange policy may be an important part of your personal value equation. Either way … I would avoid Serta and Tempurpedic and all the major brands anyway (see the guidelines here).
If you are also looking for an adjustable bed then I would make this a separate purchase from a mattress so you can first choose the mattress that is best for you and then choose the adjustable bed that has the best combination of features for the price. There is more about choosing an adjustable bed and some good sources for making comparisons in post #3 here and the adjustable bed thread it links to.
Phoenix