Socionics is a theory, MBTI is shit.
Seriously, MBTI is just an oversimplification of Jung's work (that's why it's more "popular", because it's easy to understand for anybody). When working with MBTI, people think that to type themselves they just need to choose their preference in the four dichotomies, at times without even caring about cognitive functions. Also, MBTI is full of stereotypes, which is kind of annoying (the main reason why I got into Socionics).
Socionics, instead, is a complex web of information. It's beautiful, because to discover your type, you need everything to fit the theory, this time not according to four mere dichotomies, but in terms of Information Elements, Intertype Relationships and Reinin dichotomies. Everything is linked. Most of the times that "logical perfection" can't be reached, as we're human beings and our behavior is irrational. But for this reason you need to weigh all the facts and choose your best-fit Socion. It's hard, but it's fun. And in the end, you can't even realize that you learned all of that.
Just my opinion.