You can use either solution in either case.

If you put the image on BB's dome, it will work equally well for both interior-looking-out and exterior scenes. My issue with this solution is the difficulty in getting suitable pictures to apply to the dome. They never look quite high enough res to suit me. I use the dome as a lighting solution, not as a background, but that's me.

If you put the image on a flat and scale it up to "actual" size, you may have to move it around  so it's at the right place, but it will still work. BTW... save yourself a lot of grief if you use the flat option... plug it into the "alternate diffuse" node in the material room to avoid having to worry about lighting it.