A fixed vagina is built into the dolls body.
1. More realistic feeling.
2. Better aesthetics.
1. It is harder to clean and dry after use. The doll is quite heavy, so I guess you don’t want to take her to the shower after each time. 😉
2. If something is damaged there, you can’t replace the vagina. Instead you’d need to replace the doll.
A removable vagina is an insert that can be removed for cleaning and washing (like a flesh light).
1. Very easy to clean and dry. You can turn it inside out, clean and dry it, turn it back, done.
2. You can replace it, if it’s damaged.
1. A little bit less real sensations during intercourse, as it can slightly move around.
2. There is still a risk of damaging your doll, when you put the insert in and take it out.
3. If you spread her legs, you will see the insert in her vagina.
There is no big difference. You just have to choose between ease of use/cleaning vs. aesthetics and a more realistic feeling. It’s up to you!