Hi sagvig,
The simple answer about how much quality you would be sacrificing would be “a lot”. The Lanai is a latex/polyfoam hybrid that has 2" of 1.2 lb polyfoam on top of the latex which is a low quality material that could be a weak link in the mattress in terms of the durability and useful life of the mattress. It also only contains 3" of latex on top of a 1.8 lb polyfoam base layer and polyfoam is a much less costly material than latex. There is also more about a latex polyfoam hybrid vs an all latex mattress in post #2 here.
The Spindle mattress uses much higher quality materials including 9" of latex and there are no lower quality materials or weak links in the mattress. It also has a wool quilted cover and wool is also a much more costly material and would be more temperature regulating than a cover quilted with polyfoam.
A component mattress also provides more options after a purchase to adjust or “fine tune” the comfort or support of the mattress after a purchase by either rearranging layers or replacing one of your layers with a softer or firmer version at a very reasonable cost. It also has the option of being able to replace just a single layer of the mattress if one of the layers softens or breaks down before the others (usually the top layer is the weakest link in a mattress) or if your needs or preferences change over time without having to replace the entire mattress.
Putting a component latex mattress together is a very simple job for two people. You can see their assembly instructions here and there are also some videos about assembling other component latex mattresses in post #12 here that can give you a good sense of how it works.
This topic has more information about disposing of an old mattress.
Phoenix