One of my oldest friends types as INFP - and we are very different. Jennifer does well defining the differences, and I feel like I'm just going to echo her, but I still want to see what everyone else thinks.
I would make a list!
1) He often doesn't "get" social rules, and when he gets them, he doesn't bother with them probably nine times out of ten. (Fe?) I understand social signals, and know how to respond appropriately - without thinking about it - if I need to to get something done.
2) "get something done." He doesn't seem to get things done! (closure?) He spends way more time deciding what he feels about the decision than just making it and doing something. But then - I can be too impulsive. If I think I'm right, I just do something and figure I'll deal with whatever comes out of it. But I almost never regret decisions if I know I'm right, so whatever.
3) He's far more in tune with his own value system and what he feels is right (as opposed to what everyone else around him wants/needs) than I've ever been. I'm much more out of touch with my own feelings than he is, even though I spend at least as much time wandering my inner world. Mostly I talk to the (real and imaginary) people who live there. He talks to himself there! Separating what *I* want and feel from the people I care about is something that takes work and I'm only figuring out as an adult.
Long response - and I'm interested in how much of this relates to some bigger pattern that someone who understands MBTI better can see, and how much is just my friend's and my personal quirks.
On revenge... So, I don't know about anyone else, but I'm not really aware of a desire for revenge most of the time. I try very hard to be forgiving and polite to people who violate some important idea I have about the world ("there must be a reason you suck" ), but it will build up. When I was younger, eventually I would explode at them, and they'd think I was nuts. Nice,quiet,nice,quiet... *YOU ARE THE SOURCE OF ALL EVIL*. But I don't think I really want them to be hurt. Unless they do something truly horrible. But that's different. And they deserve it.
Finally! I'm not convinced anyone fits perfectly any of these categories or we'd have 16 kinds of robots wandering the streets. Life is a complicated mix of temperament and background and who you were raised by. If you really want to find a category, I think focusing on how your mind works would be smart - that's where I really see how these two types are different. The outer behavior you have and your beliefs seems to me like it has so much to do with how and where you've lived your life.
I can be long-winded.... now you all know.