I have two lathes - a Sherline and an 11" lathe and would not be without either one. Both are fully tooled up and both are used for parts that fit comfortably into their work envelope. The advantage to the Sherline is that I can literally pick it up and put it away - it weighs all of 40# soaking wet - but is capable of very fine work on smaller parts. It also has a large enough work envelope that allows it to make parts for the larger lathe if that is needed. I cannot imagine not having both of my lathes.
In your case, your two lathes are similar enough in size that I don't see a practical advantage to keeping both, especially since the larger one has a QCGB. Yes, I agree that it is more convenient to have a lathe for different operations but if space is a real constraint then I would sell the 6" and use the proceeds to tool up the 9". Or, if the kind of work you do frequently requires you to make small precision parts then buy a lathe suitable for that purpose - a Taig or Sherline would be ideal as a second portable lathe that is cheap to own but that is fully capable of real work within its envelope.
Read the post by WR Miller above - he knows what he's talking about.