Hi lauradanielle,
dn has made most of the comments that I would have made (thanks dn :)) so I’ll just add a few.
I would agree with this and latex is the most durable of all the foam materials. There are also other factors that can affect the useful life of a mattress though that can make a difference. One of these is where inside your range of comfort and support you are and how much “room” you have for foam softening before you lose the comfort and support that is suitable for you. A mattress that is on the edge of being too firm when it’s new and has a long way to soften before it becomes unsuitable for you will last longer than a similar mattress that is on the edge of being too soft and where only a little bit of softening will take you over the edge. There is more about the factors that can affect durability and the useful life of a mattress in post #4 here.
I believe so yes but I would always check with the manufacturer of your mattress to make sure that a foundation you choose meets their warranty criteria. As you discovered … I would also keep in mind that unlike other flexible slat systems … the Laxeby doesn’t have any “flex” in the middle and this can affect the firmness in the middle of your mattress. This can be more noticeable on thinner mattresses than on thicker ones.
As dn also mentioned … it’s unlikely that these two mattresses would be similar in terms of PPP because they are more of an apples to oranges comparison. To some degree this will also depend on the layer firmnesses you choose with Spindle because it’s several mattresses in one depending on your choices.
As dn also mentioned I think highly of both manufacturers and they both compete well with the best in the industry although they are very different from each other. Spindle has a good layer exchange policy and Tuft and Needle has a good return policy so both of them provide good options after a purchase if your choice doesn’t turn out as well as you hoped.
I don’t think you will find a wool mattress in your price range (or in the price range of either of the mattresses you are considering). At best you could find a wool topper that you could add to the mattress (see the options here) but wool is a costly material and can add a significant cost to your sleeping system. It’s also somewhat unique in terms of its “feel” and will get firmer as the wool compresses (although it’s a very durable material) so I would make sure you have the chance to test a wool mattress first to make sure you like it if this is an option you are considering.
There is a list of a few futon manufacturers in post #2 here that make a wide range of futons (including some that use wool) that you could add to your research if you are interested in a futon.
Phoenix