a bad crank sensor will make it have NO spark at all, (the ECM requires 2 rotations to be "sensed" through the crank sensor, before any spark at all happens). the cam sensor (if bad or disconnected) will barely allow any spark, just enough to make you release the key from the crank position. MOST Nissan k21- k25 series motors give pretty good error condition codes, and should set a code for either of these conditions that is readable with the software, or in the dash.
Just in the last month, I have had a series of brand new bad cam sensors from TVH, (for K21-K25 Nissan motors) and resorted to having the local Nissan Dealer get one OEM that worked as it should.