I like socionics for the spin it puts on relationships. It explains well in terms of the functions how you interact with others and why someone is or isn't compatible with you. Socionics also talks about quadras. People in your quadra value similar functions and tend to be easier to get along with. Socionics seems to go more in depth with MBTI regading types. There are various subtype theories floating around in socionics, something I haven't seen with MBTI. Socionics puts more emphasis on visual identification of someone's type. I have mixed feelings about VI, I don't think it should solely be used as a typing method but I do think that certain types are more inclined to give certain facial expressions or body movements, which can be a clue.
Both theories use functions and explain the role of each one in the personality structure. Their approaches are different, not sure which one I like better, but I will say I fit INTP nicely in MBTI and INTj/LII in socionics.
MBTI on the other hand is far more widely known as a theory. It's far easier to find people to discuss MBTI than socionics. MBTI is more accessible and easier to grasp. Alot of socionics is in Russian and when its translated to English, the translation is often poor. It's harder to find good, reliable socionics tests.
I'm also active on the socionics forum, the16types.info/vbulletin I enjoy the forum and the discussions the arise there. But since there's not a whole lot of literature out there (at least the stuff that's translated in English), there's alot more speculation and conjecture. Alot of the stuff I read on that forum is quite abstract and hard to grasp without any good, specific examples. There isn't even always a good general consensus on what some of the basic terms even really mean, which adds to the confusion.