Fixed is better for experienced users and insert is better for first-timers as it is easier to clean. No difference when it comes to durability.
The fixed vagina is in-built into the body of the doll and contains texture, much like that of a real woman's vagina.
A removable vagina is an insert that can be removed for cleaning and washing (sort of like a Fleshlight). The vaginal cavity is not textured, like a fixed vagina, rather the insert is textured on the inside and it slides into the vaginal cavity. The external appearance of the vagina is the same whether you have fixed or removable insert vaginas.
More details and photos can be found here: https://sexdollgenie.com/blogs/news/sex-doll-vagina-knowing-the-ins-and-outs-of-a-sex-doll