The difference between Si and Ni is that S is linear where N isn't. They are both executed through Te.
That's why ISTJs and INTJs are hard to identify on just surface (facial expressions, posture, etc etc etc that people toy with ) without really talking to them and paying attention to what they're saying. Hardcore S types- you can almost follow their thoughts and see the connections if you are paying attention to them. Otherwise the two are very much identical. You can do this with almost any types- more similarities between types separated by N/S than T/F, or even P/J (even though they aren't really functions- they just determine the direction of the dom/aux).