Could some please explain to me:

a) What the difference is between MBTI and socionics i.e. why you can be ESTp or something on one and yet on Myers-Briggs be INTJ or something different
b) Why with MBTI, the letters are either all capitalised (e.g. ESTJ) or small if they're lesser (e.g. if your Thinking function is used about 52% you'd be EStJ), whereas if it's to do with socionics, only the last letter is lower case
c) Why some people who like socionics have a major problem with MBTI theory