I find my INFJ friend and acquaintances to be really quite different once you get past the surface. Were we INFPs tend to often come across as alternately warm or withdrawn, it seems like INFJs often come across and more cold and critical than they feel internally. While we INFPs can definitely be critical, I think that judgment isn't nearly as visible externally as it is for INFJs (unless one of our core value land mines gets stepped on).
For example, I see my INFJ coworker at work continually pushing and nudging things in the direction she believes they could go. She will sometimes ruffle feathers and create controversy in way that I as an INFP rarely do. I think INFJs tend to be more polarizing on the whole. When I campaign for a change, it's definitely done subtlety by talking to people one on one and trying to win them over that way. I think the influence of INFPs tends to be more subtle and less visible.
I also find my INFJ acquaintances to be more suspicious of motives and more likely to see inconsistencies in behavior… and even to ascribe motive to those inconsistencies, where as I tend to chalk up those things to mood and/or flakiness. I feel like unchecked Ni can lead to a kind of credulity about conspiracies and alternative medicines (although certainly not true of all INFJs... and certainly inferior and tertiary Ni can be prone to this, too). And, of course, that's not to say the Ne is always accurate, either.
Anyway, I think there's a world of difference between INFJ and INFP, even if we can have very similar values. Our functions have opposite orientations and I think that makes a big difference (J vs P means opposite orientation for every function, after all). I find expecting some mismatch/misunderstanding when interacting with INFJs helps me avoid unpleasant surprises.
EDIT: Subjectively, once you get past the (sometimes cool) surface other INFPs feel like external warmth/softness surrounding a steel core. INFJs feel like external spikiness surrounding a warm interior. To outside appearances, INFPs look softer and more flexible than they are, while INFJs look more critical and less warm than they are.