Depends on what you mean by "credible".
There are three kinds of truth: correspondential, internally consistent, and pragmatic.
We can sort cross of the second, cuz that just means the systems abide by their own rules, which, well, hopefully they do.
The first means the idea actually corresponds to reality; the third means the idea, for whatever reason, is useful.
Lastly, I should note, that I don't consider MBTI and Socionics separate: they are both Jungian typology, imo.
So, between Enneagram typology and Jungian typology, which do I find more credible?
I used to side with Jungian, and I still think there is plenty of credibility to it.
But over the last 2-3 years, I have come to find the Enneagram so damn useful, I'd have to go with it now.
I made this thread almost 3.5 yrs ago:
http://www.typologycentral.com/forums/showthread.php?t=38017
At the time, I voted that I identified with my MBTI type more than my Enneagram type.
Nowadays, I'm not so sure which I'd answer -- probably both equally.
I don't think either will necessarily gain scientific credibility.
But I think the Big 5 is a flawed instrument, and not truly scientific either.
It just lends itself more easily to methods that are considered scientifically rigorous.
I think things can be completely 100% true without science really being able to say much about them.
I don't want to say neither the Enneagram nor Jungian typology could eventually be validated or invalidated, to some extent, by science.
But the extent to which they will be invalidated, expressed as a %, with 0% being completely in validated, and 100% being completely validated... I would assume that from an invalidation perspective, it could only really ever get to ~40% (cuz the ideas just don't lend themselves too well to invalidation, I don't think). As for validation, I think they could actually become more validated than invalidated, cuz some of evidence could pop up that is close to undeniable. If some more neuroscience based stuff, a la Nardi's research, 20-30 yrs down the road starts validating some of this stuff, I wouldn't be too surprised.
It also depends on the understanding of type you're talking about: flexible and nuanced and intelligent, or rigid and dumb like [MENTION=20531]yeghor[/MENTION]'s.