Personally i don't care and boob is boob. I am a face guy so i actually have preferences to the face. Well not entirely, I'm a big guy so i would prefer my future partner to not be too much fatter then me. Plus i think actually being in love with someone would motivate me to loose my weight. But i think it would be cool if we motivated each other....any way.
As nice of a visual exposed skin can be. I actually find it more attractive if stuff is hidden. Like even during the action. I like the hair over the eyes (or glasses that obscure the eyes) and mask blocking their mouth. I don't know what kind of preference this is. But i guess its as close to a "type" i can get. Maybe i just like using my imagination.
Like this may sound...cliché? I am just a face guy and personality guy. Don't care what you look like as long as your mentally right for me.
But maybe my "boob exposure" preference will change if i find a partner. If so I'll update my awnser.