I've had an easier time figuring out my enneagram type than my MBTI type.
I find this is because my behavior changes to fit certain situations, while my motivations have been similar throughout my life.
I'm a type 6 and an iNtuitive, not an SJ. Funny, my sister is ISFJ and a type 4.
I think that the enneagram is quite a bit based on one's early life. For example, I like people a lot but I never know what to expect from them. I was teased a lot in elementary school so I still have issues trusting certain people. I also had young parents who didn't go to college and put unconscious stress on me to do well in school. I used to be a type 3 until I realized that they would love me no matter how well I did...then I began looking outside myself, developing into a 6 as I realized some sort of quantified achievement wasn't everything.
My sister felt like she lived in my shadow for a lot of her childhood, which may have caused her to develop into a type 4...If she constantly fell short of me and my achievements, she felt inadequate, so she found a way to make my parents appreciate her: by being completely different so there could be no comparison. She's also the only Sensor in a house full of iNtuitives, which could be another issue; she already feels different.
My mother is an xNFJ type 2. She grew up under an ESFJ mother who placed on her the assumption that the more you take care of people, the more they (are supposed to) love you back.
All of this is unconscious, of course. You have to look down deep to see what motivates you, and what you need. I can type my mother and sister because I've lived with them my whole life (and my mom's into the enneagram), but you can't type anyone you meet just a few times, unless they're unhealthy and their motivations are obvious.
Think of the enneagram as a sort of self-help tool. It's much more internal than the MBTI. For example, as a type 6, when I get paranoid I have to remember that I'm naturally overly anxious and over-think relationships, and my self-doubt is often unfounded, though it often seems very real. Recognizing my tendencies can help me calm down a bit.