If your sex doll is heavy and that you do not want to move her around and/or having to bring her in the shower after usage, you will need the removable vagina. I first didn’t want to have one because I thought that it would look not so real. It makes in fact no difference. Aesthetically, it looks the same.
However, if your doll is not that heavy. You may prefer the fixed vagina.
I’ll give you examples:

For my 90 pounds real doll, I use a insert (removable) vagina. The doll is quite heavy to carry around however, for my 60 pounds doll, I prefer to bring her into the shower as I personally don’t really like having to remove the vagina and put it back.

Both have pros and cons.
In summary, based on my experience with both options : If you can easily bring your doll to the shower, take a fixed vagina.