I can't speak to the future value of anything, however, I do believe the rarer something is, the more likely it is to increase in value. Thus if all you care about is a cash out at the end of the day, then perhaps you should listen to the advise of many here and go high grade.
I, on the other hand, am a collector and so I will go with Dave's advice. To me owning three nice books (lowest being a 7.0) is better than owning one 9.0 (of the same book). 7.0 is nothing to be ashamed of.
Now, if FF #48 is your all time favorite book in the whole world, then maybe I could understand you trading in books you care less about in order to try and obtain that 9.0 - 9.2.