You can install in either.
Here's how I have it:
mydomain.com/classfieds (PP Classifieds)
You see, it is very easy. Alls you have to do is set your paths correctly and it shall work.
You don't have to use the directory name "photopost" either. You can change it to whatever you want. Just be sure your path is set correctly in your config file.
Either way however, if
you have a custom skin for VB
you will need to pay attention to the image paths and menu paths. In some skins it is necessary to either hard code actual paths to images and menu items in your vb
templates and/or add the skin into your images directory in your PhotoPost product.