INFPs are more reclusive than INFJs. Under stress, they have a greater penchant for avoidance and slipping into their heads or day dreams. They'll write, they'll read, they'll take long, lonesome walks, research their favourite subject, think to themselves, examine their feelings, etc. They also don't feel overly guilty for disappearing on people because their 'alone time' is vital to their well-being. They might feel some guilt, but it's not the sort that will keep them up at night.
INFPs are also much more apt at critical thinking and creativity than INFJs are. They can out-analyze an INFJ any day, as they see a dozen solutions whereas an INFJ might be more concerned about those that pertain directly to the present situation.
INFPs also less concerned about personal appearance than the INFJ or ENFJ. Their dress is more relaxed and laid-back. That's not to say that they're slobs, they're just less likely to put too much thought into how they present themselves by comparison. They won't be slaves to fashion unless they are remotely interested in it to begin with.
INFJs, meanwhile, are fundamentally more social. Under stress, their first instinct is to seek out and help others. Distract themselves with other people's problems and you'll find them doling out advice or working on collaborative projects. They can't go days without human contact or meaningful connection the way INFPs can. When they read fiction, they're more likely to be interested in people's stories and relationships rather than the setting, plotlines or historical accuracy of what they're reading.
When approaching problems, they are more practical than worrying about the grassroots of the problem and INFJs are less likely to psychoanalyze themselves.
INFJs are more concerned with the social protocols of appearance and put slightly more effort into their looks. They are also more likely to adopt an exercise regime and stick to it because of extrinsic factors as opposed to the intrinsic motivations that might affect an INFP to take on a similar goal.
Essentially, because of their external judging function, INFJs have their feet on the ground more firmly than the INFP. INFJs are not as wont to daydream, to hermit or lose track of time and surroundings and their responsibilities the way INFPs sometimes do. Astrologically, INFJ is like a cross between Taurus and Cancer, an earth and water sign. INFP is more like water and air, a mix of Pisces and Aquarius.
Edit: Highlighted the important bits so as not to inconvenience people with 'walls of text.'