It's been a long time since I've come here last time.
Thank you for your answers.
I notice the look of the forum changed.
native speaker's intuition informs a speaker as to which one is appropriate, though few would actually put much thought into the criteria determining which to use.
here's the skinny, as far as i can tell:
crispy: used to describe things which have a stiff texture/feel on their outer covering, or which have a stiff texture throughout but are thin, primarily oriented along the x and y axes (like a rectangle), with very little depth.
eg crispy chicken (here the meat itself is soft but the skin is fried to a stiff/rough texture)
crispy bacon (very long and rectangular in shape, exceptionally thin)
crisps (meaning potato chips, stiff and brittle but thin appearing often as a circle or saddle-shape)
crisp collar (the collar of a new or freshly-ironed shirt which presents a straight, stiff appearance. note the collar is thin.)
crisp bills (freshly minted cash, still preserving the sharp rectangular corners and resistance to folding of those stacks of 'clean money' you see at banks.)
crunchy: used to describe things which have a stiff or rough texture throughout, but which are large or, though maybe small, are closer to equal in three dimensions than what is normally called crispy, or to something approaching paste or spread in viscosity but through which bits of rough pervade.
eg crunchy cookies (most are small, as in hand-sized, but the proportion of the z-axis to the the x and y is much closer than compared to things normally called 'crisp(y)' such as chicken skin, potato chips or cash bills.)
crunching on an apple (the apple is roughly spherical in shape, and more solid, physically, than crispy foods)
crunchy peanut butter (the thick, dense paste is littered with hard bits of nut)
this has come up numerous times with my foreign-born students and with my native speaker friends, which is why i've put so much thought into it.