The MBTI describes the preferences for how someone thinks - whether they tend think about subject or object (I/E), concretely or abstractly (S/N), personally or impersonally (F/T), and towards or away from closure (J/P).
The Enneagram describes mental strategies we overuse in trying to protect ourselves from perceived threats. In short, 1s strive for moral perfection, 2s replace trying to meet their own needs with trying to meet others' needs, 3s try to get external affirmation, 4s create a sense of identity based on their feelings, 5s withdraw from the world and observe it, 6s try to thwart change and establish security, 7s distract themselves from pain with entertainment, 8s try to control their external environments, and 9s create internal peace while shutting themselves off from the external world.
The MBTI is more relevant in describing a person's general outlook and how they approach situations or problems. The Enneagram is more relevant in describing how a person tries to protect themself, and how they accidentally end up hurting themself. In my opinion, it is up to each individual to decide which system is more useful to them.
Certain MBTI types correlate with certain Enneatypes, and vice versa, but you cannot directly translate from one system to the other, since they describe different tendencies.