INFJ and ISFJ are equal in value, both with specific strengths and weaknesses, but they are quite different. Perhaps the reason it seems important to raise this issue is that people don't often understand the infj, and by lumping in a bunch of other types, labeling them as infj, we get this confused mess and a bunch of people assuming they understand a type that they actually have no experience with whatsoever.
My best friend is INFJ, and I'm a strong ISFJ. I've been thinking about our differences and here's what I've found:
The major difference between us is this: I'm not living in my head while she says there's a cinema in her head and there's always something on.
my INFJ, unlike me is really introspective;
awesomely creative in terms of expressing feelings, thoughts, and impressions with words;
much more creative and arty;
she's more reserved than me;
totally lost in this world (needs much hand-holding, not practical or action-oriented in the least);
totally lost in her thoughts, dreamy, self-absorbed, spacey, doesn't seem to notice things around her. She makes me proof-read her writings because she doesn't notice typos and spelling/grammar mistakes;
depression-prone which, I think can be put down to her being a strong IN (probably);
she's prone to feeling like a martyr, agonizing about bad things that happened to her;
prone to being resentful and revengeful.
I think she's not as good at expressing empathy and support as me. (Fe?) I mean she's very empathic but when it comes down to taking actions to help ...
What we have in common is we like to moralize, we are both committed and devoted to our families, we equally like to read ... and much more like sense of humor but it has nothing to do with typology.
Very interesting thread, btw!