Cannabis is first dried and then filtered through a mesh screen. The 90/120u refers to the spacing on the screen used to filter the hash. 1 u is 1/1000th of a mm. Typically hashes are first sifted at a higher size like 200u, then through increasingly finer meshes to get finer and finer products.
120u is about as big as a nice mature trichome head will be, and 90u is about as small as a nice mature trichome head gets. Sizes below this don't necessarily result in a purer hash however, as too much below this, we start getting immature trichome heads.
The price difference comes from the fact less end product will come out of a finer mesh filter, so the economy does what it does and charges more 😀
As far as quality, I like hash. About 90u is the sweet spot and marginally worth it (even at the additional cost) compared to a 120u.
This being said, bubble hash, which uses frozen cannabis (not dried) and a similar sifting process, extracts more of the good stuff .
