I won't waste much time on the Temperament model since INFPs and INFJs both possess the Catalyst Temperament -- which contributes to uncertainty, since it describes what they have in common, rather than how they are different. (Note: I am utilizing the newest names for the Temperaments as chosen by Interstrength Associates and Dr. Linda Berens.)
Now both INFP and INFJ often test or mistake themselves for INTJ, so this is a good model to help sort that misunderstanding out. (For a more in-depth comparison of INFJ and INTJ, I have a page available here that includes video and a link to a 1-hour audio interview of an INFJ and INTJ discussing their type differences.)
Since the Temperaments are quite different between the INF_ and INTJ codes, taking a close look at core Temperament values tends to be the most insightful way to sort through this confusion. (It's also useful to evaluate the dimensions of pragmatic/affiliative, and the language preferences of abstract general vs. abstract specific.)
A Theorists' motto might be "Cogito ergo sum." In contrast, the Catalysts' motto might be "Sui generis" -- unique, one of a kind. Which one of these mottos resonates more with you?
Using the type model is not the best way to figure out INT vs. INF.
INFPs and INTJs both feature the Te/Fi axis, which creates functional confusion. INFPs possess extraverted Thinking in the aspirational position, which may delude INFPs into believing their Thinking is more proficient than it is. Since this function is the INTJ auxiliary process, it is easy to suppose the INTJ code reflects their personality better than the INFP code. As a matter of fact, I wish I had a nickel for every person I've met who claimed to prefer INXP for the same reason. But that's the only way some INFPs can account for what they think is an equal ability to access F and T. (I often hear this justified through a right-brain/left-brain rationale, which is coincidentally a Te way of thinking about this paradox.)
In contrast, the INFJs' devotion to time and task may cause them to look like INTJs. They tend to prize getting the job done over caring for the people. (How many conversations do INFJs cut short so they can get a task completed on time?) They display their judging function to the world, which can seem harsh on occasion, and thus may be mistaken for Thinking. Moreover, as a function, Extraverted Feeling strives to be objective (per Jung), so sometimes INFJs seem like "idealistic" INTJs. (I've seen Catalysts characterized as "Romantic idealists," and Theorists as "Pragmatic idealists.") Because INFJs and INTJs have the same interaction style and both tend to be reserved, it's possible for INFJs to test, identify, and consider themselves to have preferences for INTJ, especially if they equate Feeling with "sentimental."
If these issues are anything you're struggling with, an honest, thorough investigation of Temperament should help sort matters out.
Of course you will identify with parts of every Temperament description -- that's normal. But one of the patterns fits you best!
Take care when investigating Temperament not to be "additive" about the descriptions. Go at it with a mindset of what you can't live without. In other words, Theorists are not people who crave a unique identity and meaning and significance in their lives who coincidentally are trying to accumulate knowledge. And vice-versa for Catalysts! You must decide which values you can't live without. Let's face it -- many people accumulate knowledge and seek meaning in their lives. The key is to determine what you must have in order to live. As Dr. Berens says, Theorists would be psychologically dead if they could not pursue knowledge, and Catalysts would be psychologically dead if they could not pursue meaning. So don't consider what you would like -- consider what would destroy you if you couldn't have it anymore (or you pursue it so reflexively that it's automatic).
When exploring Temperament, pay close attention to the pragmatic/affiliative aspect and the structure/motive aspect, because these dimensions clarify Temperament. Catalysts are affiliative/cooperative (not independent/pragmatic) and interested in motive (the reasons why people do what they do). None of the other three Temperaments does both.
Having said all that in order to distinguish Catalysts from the other temperaments, let's take a closer look at the INFJ and INFP pattern through the lens of temperament. Let's take a deeper look at Temperament that most people don't know about.
Within the general Catalyst Temperament, INFPs and INFJs show different "colors" and have different aims.
INFPs are known as "Harmonizer-Clarifiers." "Harmonizing" is how they see themselves, and "Clarifying" is what others notice they do.
INFJs are known as "Foreseer-Developers." "Foreseeing" is what they see themselves doing, and "Developing" is what others notice they do.
INFJs tend to be "people-builders," while INFPs tend to be "culture-builders." INFJs get invested in particular individuals and seek to help them develop, while INFPs often try to control the environment and strive to ensure the "atmosphere" is harmonious and conducive to growth.
We'll talk more about INFJ and INFP differences further on -- because that's the whole point of this website!
The following book is invaluable for exploring Temperament: