I prefer the 3 letter code just because it makes a clear distinction between socionics and MBTI (especially here in America where MBTI is widely known). Speaking socionics in 4 letter codes also makes it erroneously seem like types can be converted from one system to the other through whatever formula. My experience has revealed to me that typing socionically is way different from MBTI typing, and any similarities in letters between the 2 are purely coincidental.

That said, my preference is not THAT strong, as long as people understand the distinctions. My preference is more so as not to mislead the beginners than anything else.