INFJs have the Catalyst temperament, while INTJs have the Theorist temperament. Their core needs are different, their values are different -- and so are the skills and behaviors you might see in the outer world. Just a little bit of delving into the differences in Temperament is the fastest route toward discerning which type pattern is the better fit. (Brief Temperament descriptions and their core needs may be found here.) INTJs tend to display a scientific bent, while INFJs tend to display a humanistic bent.
An important distinction between INFJs and INTJs is that the "intelligence" or "skill-set" INTJs excel at is Strategic, followed by Diplomatic; while for INFJs the order is reversed: Diplomatic, followed by Strategic. So INTJs typically consider Logic first, people second; while INFJs tend to consider people first, Logic second.
INFJs and INTJs share the same interaction style, which contributes mightily to the misunderstanding -- however, there are slight differences within the way the style gets expressed. But if you're new to type, those distinctions are likely to be too ambiguous to comprehend at this stage of the game. Either way, both type patterns will naturally "anticipate" (which is sometimes conflated with either Strategic or Diplomatic intelligence in the Temperament model).
If one relies on the MBTI dichotomies, it is all too easy for "Fs" to imagine they are "Ts", and for "Ts" to imagine they are "Fs." The only way to get beyond this misunderstanding is to delve into the eight-function model and explore the cognitive processes in greater depth.
Here again, as with the Temperament model, the differences come into better focus.
INFJs and INTJs share the same dominant cognitive process: introverted iNtuiting (Ni). This is commonly called "foreseeing." Thus, both INFJs and INTJs are both interested in synthesizing, integrating, conceptualizing, understanding complex patterns, and focusing on the future.
Where the divergence occurs is around the auxiliary process. Here's where Thinking and Feeling pop into view.
INTJs have a preference for extraverted Thinking as their auxiliary process. Thus, they are given to "segmenting" -- which includes measuring things with objective criteria. They will naturally calculate how long something will take, how soon the next event will occur, and enjoy telling you about these measurements. They like organizing for efficiency, systematizing, structuring. Their tendency is to provide you with their rationales: "The reason I'm late is because it took me 15 minutes longer on the freeway than it normally does. I was only able to maintain a speed of 25 mph, compared to my normal speed of 40 mph." And so forth. These rationales and measurements are very important to INTJs. They especially love to predict how long something is going to take, and their wristwatch is probably among their prize possessions. They collect trivial facts constantly, and can probably tell you all the contents of their medicine cabinet -- including the proper names for all their pharmaceuticals. Just ask them about their computer -- they'll tell you the processor speed, memory size, hard disk capacity, operating system version, and network capabilities. Chances are they've even memorized the Periodic Table of Elements.
INTJs like to work with challenges that occur in the moment -- the ones right before their eyes. If you want to see an INTJ in top form, watch them respond to changes in travel plans. If a flight gets cancelled or delayed, they will rapidly move from Plan A to Plan B, and so on, until they exhaust all possible contingencies. Even if they fail to salvage the situation, they relish trying to solve the immediate problem. They like to "get their hands dirty," so to speak. If there is a computer malfunction, they are inclined to grab the mouse and take over the keyboard so they can start "trying things out" right away to fix the problem. They can clean a house faster than a tornado if company's comin' -- just don't expect them to remember where they put things after company goes home!
INFJs have a preference for extraverted Feeling as their auxiliary process. Thus, they tend to be "charming" and are given to "connecting" -- which includes maintaining an extensive web of relationships. They are given to adjusting to and accommodating others, deciding whether something is appropriate or acceptable to others. For instance, one INFJ experienced a great deal of stress on a message board when some political bashing started. There was an implicit assumption that everyone on the board was a member of a particular party, when in fact this INFJ knew members of the opposing party were present. It caused her no end of stress that these people were being maligned by the bashers, even though there was no apparent conflict. Other INFJs maintain internet blogs and "connect" with others in cyberspace, and maintain their web of relationship via that means. If she were to dine with others, she would be concerned that the food appeal to everyone's tastes, there are no food allergies, and that the price is within everyone's budget (if it's a restaurant).
INFJs sometimes speak in the "Royal We" -- meaning they feel qualified to say what the group needs or wants. They may take on the role of spokesperson for the entire group, and experience the group as an "entity" unto itself (rather than a collection of individuals). At the same time, INFJs typically know what each member of the group personally likes or dislikes. She knows who's married to whom, who's pregnant, which friend has the deadbeat brother-in-law, which cousin is the reformed alcoholic, and whose mother just died a month ago. She knows this data off the top of her head, and can rattle it off without a blink. (If she's reading this now, she probably thinks it's no big deal -- doesn't everybody do that?) If it's not this kind of information to do with relationships, then it might be about something similar, such as how others like their clothing, or their artwork, favorite colors, or whatever peccadillo is unique to that individual.At Christmas, INFJs are good at picking out suitable Christmas gifts and remembering which niece likes Dora and which niece is into Cinderella. INFJs can be sponges for such trivial data and habitually collect them (often unconsciously) so they can better "consider others" with the information. Sometimes they are more protective of a person's likes and dislikes than the person is themselves!
Moving through the personality pattern, we now come to the tertiary process. And it is precisely this process that often confuses people as to whether their preferences are for "F" or for "T," because it will be the opposite of whichever was their auxiliary. Take careful notice about which of the following descriptions fits you better.
In the INTJ pattern, the tertiary process is for introverted Feeling. Thus, they habitually consider importance and worth, clarify values, and decide whether something is of significance and worth standing up for. ("This doesn't match our priorities so we should refuse that assignment.") They have a strong sense of what's important to them or the project. Sometimes they can be seen as "selfish."
In the INFJ pattern, the tertiary process is for introverted Thinking. Thus, they habitually analyze. They devote energy to deciding whether something fits a framework or model, and will often clarify definitions to get more precision. ("Let's define our terms before we get more deeply involved in this conversation.") They enjoy categorizing, and figuring out underlying principles. Sometimes they can be seen as "critical."
It's been my observation that naive clients sometimes find "truth" in their MBTI results, even when the descriptions don't fit perfectly. It's a touch of the Barnum Effect -- and is entirely understandable. After all, the assessment got so much right -- it seems unreasonably nitpicky to reject some of the details, especially when a misidentification around the auxiliary can usually be *stretchhhhhhhed* to fit the tertiary. Thus, it's extremely worthwhile to take a close look at both the auxiliary and tertiary processes to match how well they truly fit and match your real-life experience. Is your Feeling preference introverted or extraverted? Is your Thinking preference introverted or extraverted?