Let's make my point clear then.
Based on wikipedia MBTI pages, cognitiveprocesses.com, typelogic.com and humanmetrics.com and in fact any other official MBTI or MBTI look-alike site online I've read so far, including Keirsey (and please if you want to tell those are not pure MBTI and are incompatible, then post concrete evidence that those differences are more than nitpicking around terminology and focus differences in application area and tool purpose) They all affirm they come from a Jungian heritage. All.
I'm going to talk about the j/p switch and function order difference between all these systems and socionics. What I call j/p switch is only needed for introverts/intratims (so for I types in both theories) meaning for instance that INTp becomes INTJ and INTP becomes INTj.
1) I gathered from the maximum number of sources stereotype "nicknames", in both camps. And when reaching something like 10-15 7-10 "nicknames" for each type, the j/p switch is clearly obvious for all introverts. Anyone having a minimum amount of intuition would get it just by reading those series of words and trying to group them together (I'm sure even a kid could do it), and as it's a really "light" point, discussion-wise, I won't even bother giving you an example.
2) One system is focused on functions, the other on information elements. But what are those information elements if not inputs or products of those functions? Simple example for the Feeling function (taken from MBTI auxiliary Fe description) "Fe seeks social connections and creates harmonious interactions through polite, considerate, and appropriate behavior." Alright, what is this talking about if not ethics valuation and production? Aka "what I feel about something, is it good? does it make me feel better?"
3) Also thinking in functional terms is perfectly valid socionics. But on wikisocion one can read: "According to Model A, there are eight functions present in each type, one for each information element. In this respect socionics differs from canonical MBTI and Jung's Typology, in which only four elements are present in each type.". This is in contradiction with *modern* MBTI: "Later personality researchers (notably Linda V. Berens)[24] added four additional functions to the descending hierarchy, the so-called "shadow" functions to which the individual is not naturally inclined but which can emerge when the person is under stress." So you can see both systems have same 8 functions and both have a notion of "conscious functions" and "unconscious functions" (even if in detail and order they differ)
4) Even by not going too deeply into said details, when applying the j/p switch for introverts, nobody can argue that the two first functions (MBTI: 1) dominant, 2) auxiliary, socionics: 1) leading, 2) creative) suddenly match for all 16 types. Ex:
INTP = INTj = Ti Ne
INTJ = INTp = Ni Te
5) No need to apply the j/p switch for extroverts, they already match:
ESFP = ESFp = Se Fi
ESFJ = ESFj = Fe Si
(you can verify for all 16 types if you don't trust me)
6) Intertype relations do transpose 1:1 between both systems when applying the j/p switch, and guess what, it's exactly what typelogic.com did! They just renamed "duality" into "anima/animus"...(I hear you cry "but it's different! it must be different!") Well no, having studied both, it's the same but different wordings, inter-type relations wise. Also no need to point out how commercial dating sites are leeching socionics intertype model to provide their advices on top of MBTI/jungian derivatives (but some don't take too much risks and only propose people "identity" relations because it's safer and helps them say they make people date easily whereas they don't care if they'll stay together or not..)
Now onto the differences, because it couldn't be too easy either:
1) About functions balance in MBTI I'll quote this: "Jung and Myers considered the attitude of the Auxiliary, Tertiary, and Inferior functions to be the opposite of the Dominant. In this interpretation, if the Dominant function is extraverted, then the other three are introverted, and vice versa. However, many modern practitioners hold that the attitude of the Tertiary function is the same as the Dominant." Meaning with INFJ as example:
Ni Fe Ti Se -> Two inputs, two outputs, one of each kind, looks very balanced and efficient to handle life and survival
2) In socionics with same example it's unbalanced, in the sense that both "captors" (inputs) are introverted and both "actors" (outputs) are extroverted:
Ni Fe Si Te -> Two introverted inputs, two extroverted outputs, makes a very nice "information processing vector"... I could translate that into a cellular automata and each type would be a "glider" always going into the same direction (nice concept though, but a bit akin to walking with one leg half the size of the other...)
3) But then, who cares, maybe it's just me in this case nitpicking about function orders, they probably have different definitions for each of them (each "slot/position"), and also what one system consider "cognitive" functions could be different from the other which considers things more in terms of "conscious" or "unconscious" processes...To be honest, I don't know.
4) My current hypothesis is that MBTI allows to test and see the person's EGO and SUPER-ID blocks in model A (with my IEI example: EGO: NiFe SUPID: TiSe.....well SeTi, but no nitpicking I said!)
5) Then a last example with IEI duality this time: When you have to explain in the most simple way to someone why duality "ticks" and why conflict is a "conflict", what would you choose to draw on that napkin in front of you:
a) using MBTI functions notation
IEI + SLE = Ni Fe Ti Se + Se Ti Fe Ni =
IEI + LSE = Ni Fe Ti Se + Te Si Ne Fi =
b) using socionics functions notation
"
- Well you have four blocks in your mind, ego, super-ego, id and super-id (while drawing squares on napkin...) and your ego rings something in your partner's head...here...(while pointing at little square)...and..
- Come on! Get to the point I hate when people turn around things and are not clear! (said the SLE)
"
PS: If any of the information sources I cited, including wikipedia and wikisocion, or any other site is false or erroneous, what are we waiting for correcting them? Leaving falsehoods online is misleading to people and we must do our possible to prevent that. And don't forget to paragraph!