First of all, overall rider height isn't the critical measurement for bike sizing. Inseam is. Measure your inseam and see how that compares with other riders.
For instance, I'm 6'0", but I've got short legs, so I generally feel comfy on a smaller frame with a longer stem. Someone who is 5'10" with longer legs might be more comfy on a larger frame with a short stem.
Ask the shop to let you try the 17" with a slightly longer stem.
In between sizes on a mountain bike, I would generally steer towards a smaller frame. Too many instances where a lower top tube can save the "boyz" if you know what I mean.
Also, a smaller frame is stronger due to the shorter tubes making up the frame.
Are you locked into buying a Gary Fisher? Have you tried other brands?
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