The two INT types are extremely similar. Is the INTP more like the ENTP or the INTJ? The answer might be either one depending on what criteria you are using. The ENTP and INTP use the same functions, so they process things in an extremely similar fashion. However introverts and extraverts ultimately have different goals, motivations, etc... which are most important to them. That is why you can say the INTJ is the most similar to the INTP, because INT's are ultimately interested in the same types of things, but they process information and therefore achieve their goals in totally different fashions.
For example both INT types are interested in developing new (and desirably correct) ideas. But how they develop these ideas tends to be totally different. An INTP might read a wide variety of sources and listen to a wide variety of people to generate hypotheses, but internally each one is carefully weighed logically and compared to the framework of conclusions that they have already reached. In this way an INTP slowly and carefully builds an elaborate mental framework relating to the subject he is trying to develop.
In contrast an INTJ might come up with an entirely novel idea on their own, but then they need some way to test how true it actually is. So then they might look up what the latest research on the idea is, and then reach a conclusion and go onto the next thing. Or alternatively they might perform some type of experiment (possibly even on themselves) in order to reach a conclusion.
So in general INTJ's and INTP's are very similar in what they want, but totally different in how they go about getting it.
For example both ENT types are attracted to abstract ideas, but they are much more interested in applying these ideas to the real world than INT's are. ENT's want their ideas to externally achieve something, but the ENTJ leads with thinking and uses intuition for support, while the ENTP is opposite leading with intuition. So when achieving a goal an ENTJ might first look for the most efficient method possible, while the ENTP might look for the most original method possible. (Of course it's possible that in looking for the most efficient method the ENTJ develops a totally original one, and likewise the ENTP could invent the most efficient method by starting with a novel approach.)
Ultimately why the ENT types do what they do is very similar, but the how tends to be totally different.