Good question. I don't think I know any INTJ's. I'm willing to bet that the INTJ's like to read poetry and have a better understanding of school and things like that whereas ISTJ's are probably more "hands on".
One of my closest friends is an INTJ male. On the outside, you'd probably think we were the same person...it is uncanny how we react to things in very similar ways.
Most of the time our thoughts are even in sync with one another (but that could just be b/c we've known each other for about 8 years). Of the differences I see between us, he is very intellectual at pretty much all times. He enjoys reading informational texts on all kinds of stuff, whereas I probably won't read about it for enjoyment...I'm more likely to just jump strait into a project, ask for help, and/or figure something out by actually doing it.
His thought processes are probably more fluid and changeable compared to mine...he expects things to go wrong, and will adjust accordingly. He's pretty good at improvising things -- me, not as much. Teaching has taught me to be flexible, but I still like being planned and rehearsed and decided on things ahead of time. He sees the "big picture" more easily, but I'm the one that catches the details.
Socially, I'm more outgoing and talkative, and can tolerate people a little better. He prefers to keep to himself, and shows very little emotion. He has 2 default facial expressions: smirk, and neutral. It is rare to see a wider range of emotion than that (could be a guy/girl thing, though?).
If you compared an INTJ and an ISTJ, you'd probably notice the following things:
The INTJ is likely smarter
The INTJ will likely focus on the bigger picture, while the ISTJ will more likely be inclined to focus on the smaller picture
The ISTJ is likely to be a harder worker
The ISTJ likely has more respect for authority
That being said, those characteristics would be the best for telling the difference between the two types.
Ri-ight. So intelligence is suddenly influenced by type. Although there are patterns it's not enough on any scale to really help determine effectively a persons type. I do believe that other factors are much more reliable and that the first suggestion for making distinctions is a poor one. Everything is based off of the individual and we try to fit it into type. It depends on what has influenced your personality for some of these factors more than what your personality is.
"I am what I need to be..."
"Nemo me impune lacessit - No one provokes me with impunity."