From what I have noticed, ISTJ's are more concerned with tradition and are much more conscious of anything being right or wrong. They are also very dependable, more so then most types, ( generally speaking )
INTJ's on the other hand are more interested in intellectual pursuits and are considered to naturally be more open minded. They are very confident in themselves in leading others (sometimes a weakness)
Both types generally behave very similarly, ( to the untrained eye ) but on the inside they orient themselves to the world in completely different ways. where the ISTJ is more concerned with there literal understanding and where the INTJ is more concerned with their figurative understanding.
I hope that helps.