I say intake.. Yes what you heard is right about the inconsitent times.. It all depends on the bottle temp and pressure- which sucks cause i found that every shot i would go a lil bit slower..
i had n20 once (zex kit) and the kit was good, made good power, but always left an empty feeling in my stomach- kinda felt i was "cheating".. If you dont have heads and cam, i would do that before wasting money on n20.. Filling the bottle often gets very expensive.