I don't think I have ever used the makeup vanity I have. I would definitely go for more storage space with a linen closet. As of now, my linen closet is in my hallway, and when I forget to grab a towel...it can be a pain ;)

On a plus side, instead of doing a built in makeup vanity, you can always purchase one that can be moved from bathroom to closet or from old bathroom to new bathroom. I think that would be a much better option. Or a small childrens desk could work too, just sand it down and add a fresh coat of paint.

Definitely more storage.

Storage

Storage!

I've never heard a buyer on House Hunters say anything about a bathroom not having a make up vanity. They do often mention if one doesn't have much/any storage space tho.

Is there room for a closet door to swing open? You could just do open shelving -- even glass shelving with lighting above for a more open look. Doesn't have to be full depth of the tub. It sounds like you already have 80" of under-vanity storage.