Right I'm thinking along the lines of MBTI and Enneagram. I've known a few people who when doing soul searching found one or the other more helpful.
Basically I'd like to discuss what one represents as opposed to the other, how much they should be integrated and what dangers, hazards and highlights there are to this kind of approach.
Short answer:
MBTI is an analysis of our priorities when we gather and process information. These priorities generate the sixteen types.
The Enneagram is a list of nine archetypes as the basis for the system, rather than starting with foundational functions. The types are more closely bound in the sense that people who are either maturing or declining will drift in defined ways towards one of the other archetypes. It is this sense of "movement" within the theory itself that probably makes it appeal to the more spiritualized audience.
Some aspects of the MBTI and Enneagram do intertwine. For example, some of the types commonly map into the other system. The INTPs are usually Fives, for example, and occassionally Nines. With the Fives, I am going to hazard a guess that 5w4 INTPs have more developed Fi/Ni/Ne (one or more of those) and not as stringent a Ti sense as the 5w6, which seems to be more about Ti and Si development. A case for stronger Ni in the case of the 5w4 can most easily be made because INxJ types (with Ni as their Primary) map most often into the Fours. Also, interestingly, the 5w4 wing has been identified as the "Iconoclast" (the Enneagram type that deconstructs "signs"), and if you study the Ni function (see the Lenore Thomson wiki), you'll see that this is exactly what Ni does... it recognizes that signs really do not have inherent meaning and thus steps outside them in order to better see what is happening.
[Thoughts on this are appreciated.]
In this sense, I find a combination of MBTI/Enneagram to be useful to explain variances within types in MBTI. You also see potentials for growth that you wouldn't find in MBTI. For example, INTPs are told that they need to develop Fe to eventually mature, but the Enneagram takes a different tact: Five goes to Eight, which means the INTP grows by involving himself and trusting his instincts (TiNe working together) rather than sitting back detached and uninvolved in life. Those are both good strategies, and MBTI doesn't really engage the latter one.
One thing to be careful of is that there is no good map. Some types map easily, others do not. There have been numerous theoretical approaches as to what the map should (I've personally seen at least 4-5 different ones).