It does depend on your luck if REI has someone with a clue. I'd take the equipment you expect to carry if at all possible. They do have sand weights, but of course they carry different than actual "stuff". I did have better luck finding someone who had knowledge at the local outfitters. You do need someone who knows how to fit the pack - maybe call ahead and see when a good time to stop by is? Or learn how to fit the pack yourself. For instance I know that there are several different sized hip belts in the Osprey line, so I could ask do you have the smaller / bigger one and then feel the difference. However it was nice to have someone there to make suggestions, such as I wear my pack incorrectly (which prior to fitting I didn't know) - I wear my hip belt on my hip bone tips not the iliac crest where it is supposed to be.
Also, just a personal opinion, I (ummm) sometimes listen better to someone else helping rather than the poor hubby.