The obvious thing to look at in comparing the two is the introvert/extravert difference, but there’s more to look at than just that.
There are some notable implications of having different sets of primary & inferior functions, for one. To start with, INTJs lead with an introverted perceiving function (Introverted iNtuition), while ENTJs lead with an extraverted judging function (Extraverted Thinking). As a result, INTJs are going to be slower to act and make decisions. While ENTJs are rarely going to be reckless, they’re going to be more execution oriented and they’re going to tend to take action faster. One of the ways this shows up is that ENTJs will tend to get out to an earlier start when it comes to their careers. A teenage INTJ might spend an inordinate amount of time indulging in deep philosophical questions about life while an ENTJ of the same age might already own a record label with 8 people working for them.
Of course there are exceptions to this and any other rule in telling them apart, but generally left to their own devices, ENTJs will tend to get traction in life a little faster. INTJs are more likely to be late-bloomers.
Another consequence of the difference in primary functions is that INTJs are more likely to have difficulty expressing themselves, especially at a young age. Introverted iNtuition is a function that runs deep into the unconscious, and while this aids them in creativity, it can be more difficult for them to formulate their thoughts and justify their positions. In some cases this can cause them to be withdrawn or to develop anger issues. Another consequence of this is that a much higher percentage of INTJs turn to outlets like music to express themselves and therefore, there are many more INTJ musicians than ENTJ.
Naturally, with the differences in primary functions, there are going to be differences that come from the opposing inferior functions. The inferior function is generally more difficult to access and control than the tertiary, so ENTJs are going to be more awkward in the emotional realm and INTJs are going to be more awkward in the physical realm.
ENTJs can have a tendency to be more detached from their emotions. They’re more likely to act without empathy. When they’re triggered emotionally they can become childish and tyrannical.
INTJs can get hung up on their intuition and get stuck in their heads, not seeing what’s right in front of them. When they do engage their senses they might be somewhat clumsy and in some cases even self-destructive. They can plan many moves ahead using their Introverted iNtuition and Extraverted Thinking, only to undo themselves seeking moments of fleeting physical pleasure through drugs, alcohol or other means.
If you’d like to learn more about how to tell the difference between INTJs and ENTJs with video examples, you can check out the INTJ vs ENTJ video in the “Comparing The Types” series here: INTJ vs ENTJ.