I'm a little confused by this, because you think they are either ISTJ or INTX. To me, that really means that you are unsure of their type and believe them to be IXTX.
It's not easy to determine someone's type correctly. Most of my friends IRL that I have determined their type through observation, have been close friends for over 5 years. The simplest thing to do is to get to know them better.
I'm really leaning to ISTJ or INTJ. INTP is definitely iffy. This person isn't very flexible or spontaneous at all. The INTP's I know have a certain amount of fluidity in their interactions compared to ISTJ/INTJ so I'm thinkin' it's gotta be between ISTJ or INTJ....probably ISTJ because the few interactions I've had didn't feel like anything special. I usually connect inexplicably well with other NT's.