Hi Pegabo,
Jeff already made some good comments about the adjustment period which would be part of any new sleeping system and there is also some information about the break in and adjustment period in post #3 here as well
I certainly wouldn’t purchase a mattress that your testing didn’t give you some reasonable confidence that it would be a suitable choice for you and if you aren’t confident that it’s a good “match” for you in terms of PPP then I would make sure that you are comfortable with the options you have after a purchase to make any fine tuning changes to the mattress that may be necessary or to return or exchange it just in case your choice doesn’t turn out as well as you hoped for.
I would also keep in mind that you can always add additional softness and pressure relief to a mattress that is too firm by adding a topper but it’s much more difficult to “firm up” a mattress that is too soft without removing and replacing the layers that are too soft so as Jeff also mentioned when you are considering two mattresses that both seem similar then it’s generally best to err on the side of the one that is slightly firmer.
Post #2 here has more about the different ways to choose a suitable mattress that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them.
Hopefully you will end up with a few good “finalists” to choose between and I’m looking forward to finding out what you end up deciding.
Phoenix