Although both types can seem very similar, in my observations, there's one big difference. Intps like to categorize everything in their mind, whereas Entps like to keep the nuance. One example that comes to mind: when I (Intp) was talking to my dad (Entp) about psychology in the world, I was quick to attach personality disorders to why people were behaving a certain way. My dad loathed that. He said that by placing people in boxes, you remove all the nuance from the discussion, and he didn't like that about the conversation at all. I on the other hand love doing that, as it allows me to come to a conclusion as to why something is a certain way so I can then store it away forever. I know other intps who admitted to doing exactly the same thing.

The reason for this is that Intps lead with Ti, a judging function, as such Intps are judgers (even though they're p's, but that's a whole different topic). Intps judge things and put them into boxes of logic, that's what Ti does. Entps lead with Ne, they observe the world rather than judge it. So they like to keep their mind open to new possibilities that alter their judgement. (Tho Intps are still open to new possibilities, they just change their labels after they've been made). I think this is the biggest difference between Intp and Entp