Personally, I would opt for the larger space detached from the house that would be all my own. I've learned that having your own space can be very nice. When you're using part of another space it's easy for things to distract you. Its also easy for your stuff to encroach on the rest of the space.
How often do you have guests? It might be just as well to use a blow up mattress or have a sofa bed in another room for them if you don't have them often.