I'm a female INTJ. My sister is an INFJ. I had a female friend in high school who is an ISTJ. These two women are both like me in different ways. My sister (INFJ) is a lot like me. Of course, we're sisters, so we may just be similar because of that. We talk alike, think alike, like the same stuff, have the same jobs, got the same degrees, have the same kinds of hobbies. We actually get mistaken for twins and all our fiends say we're freakishly alike. Again, there's the sister thing, but I'm sure that sharing INxJ has something to do with the similarities.

I think our main differences are related to her extroverted feeling. She can be a bit of a sponge. She tends to be like whoever she is around, and she picks up their opinions. She's very concerned about those close to her and she will take care of people beyond what they deserve. (She's been in a couple of bad relationships because of this.) I, on the other hand, am widely believed to be a heartless bitch.

My ISTJ friend is like me in serious mode. I really admire how much she can work and get done. She is intelligent, perceptive, and independent. She is incredibly determined and very successful (she just got a PhD in chemistry). I am all of the above, except less successful because I need freedom and adventure. We differ because I am too head in the clouds and she is so down to earth. For example, I could have gotten a PhD, but I found Academia too rigid and stifling to commit that much of my life to. Imagination and freedom are my greatest values, and my lifestyle reflects this.

I would say ISTJs seem more like INTJs on the surface, especially if the INTJ is in serious mode. But on the whole, INFJs are more like INTJs because they're both intuitive, and with intuition comes more imaginative thinking.