mountain bike fit?

[VW Beetle]

I recently bought my first mountain bike, and although nothing is actually uncomfortable, I am beginning to worry that I should have purchased the smaller size. In particular, if you are new to mountain biking, your roadie instincts about proper fit may be very misleading.

I am 5'7 with a ~31.5"-32" inseam (long legs, short torso). The two LBS guys helping me with my purchase disagreed about whether I was a better fit for a 15" or a 17". I went with a 17" WSD Rockhopper, which actually has the reach of the 15" men's model, but the height of a 17" model. At the time I purchased it, I thought the reach was very close (almost too close) and my greater concern was that I might be buying a bike that was too small. (My understanding is that Rockhoppers are known as relatively short reach bikes, in contrast to Gary Fisher and others known for their longer reach). I have 3" of standover clearance and 6.5" of visible seatpost. I am not uncomfortable in any way, but I do have this nagging suspicion that the 15" would be more chuckable and nimble, as others have aptly put it.

PS: I have most often read 3"-6" of standover clearance is ideal, with standover measured by standing astride the bike, lifting it by the seat and bars, and assessing how far from the ground the wheels are.