INTP and INFP the same? Exterior wise I'd say yes but it's often noticable that INTPs are a lot more concerned with definition than INFPs. An INFP is more likely to say something like "cause it does" than an INTP.
Based on my two INFP mates it would appear that I like to question a whole lot more than they do. Sure they'll complain about things but when it comes to analysing stuff and finding a point where I usually go "why can't they just..." then I'm far more likely to do that than to complain about the end result.
Another difference is in communication. I'll engage your thinking where as they tend to engage people's emotions more. It's most noticable in our persuasion methods. Often they will engage in that kind of persuasion where things like "well if you're not bothered about it then fine" are said where as with me I'm more inquisitive and will engage your reasoning in some conversation (some say argument... but they're just plain wrong
).
I guess that the central difference is pretty much summed up by the differences between Ti and Fi. One INFP friend tells me he has almost a logic machine in his head where he can feed in the input and see what this machine says should be the logical outcome... me I've got a feeling machine where I calculate out what impact my actions may have, for example, by testing various hypothesis in this virtual environment I create.
Mind you though I've found few developed INFPs who I don't get on with... the immature ones are only slightly less annoying than the immature INTPs.. I guess that means they are, in effect, quite similar.