I admit that I haven't spent much time on eiter ELL and ELU and I haven't done any systematic research, so this post will be based on anecdotes rather than statistically significant samples.
The difference in terms of official scope between ELL and ELU is minimal. The following topics are listed on both sides:
- Word choice and usage
- Dialect differences
- Spelling and punctuation
ELL adds “Practical problems you encounter while learning English” — whatever that is supposed to cover. (Maybe “practical problems you encounter while teaching English” would have more semantic content.) It's unclear to me how such problems would be off-topic on ELU.
ELU adds “Pronunciation (phonetics and phonology, dialectology)”. I don't see how these topics would be off-topic on ELL (perhaps the intent is that ELL would focus on usage while ELU would tend to veer more into linguistics). ELU also adds “Etymology (history of words’ development)” which is explicitly excluded from ELL. That's a pretty minor difference all things considered.
This leaves the audience as the sole differenciation factor between ELL and ELU. ELL is for “people who are learning or teaching English as a foreign language” while ELU is for “linguists, etymologists, and (serious) English language enthusiasts”. The upshot is that threads on ELL are more likely to veer towards teaching to non-natives (with more formalized grammar, special attention paid to false friends, etc.) while threads on ELU are more likely to veer towards scientific concepts from linguistics.
In summary the two sites accept essentially the same questions but invite different kinds of answers. This is not necessarily a bad thing. There are precedents with Stack Exchange sites whose scope significantly overlaps but that target on different audiences, however in all other cases the ground covered by one of the sites is significantly larger than that covered by the other site.
For example, Unix & Linux is mostly but not exclusively a subset of Super User and is a superset of Ask Ubuntu. AU in particular tends to invite different kinds of answers from U&L for similar questions (one site caters more towards novice Ubuntu users while the other caters more towards advanced users, not unlike ELL vs ELU — however the difference is more subtle than this). U&L vs AU is however a special case in two ways: Ask Ubuntu has official ties with Ubuntu, which would not be possible with a less focused site; and the volume of AU is about 5 times that of U&L and roughly equal to that of SU despite the scope hierarchy.
In the case of ELL and ELU, do the different leanings really matter in practice? In my experience, not really. A majority of posts on ELL involve no pedagogy and a majority of posts on ELU do not involve linguistics, so they would be suitable for either site.
The main difference I've observed between ELL and ELU is what happens when a question can be answered either with a simple answer (“phrase A is better than phrase B”) or with a detailed analysis (showing other possibilities, discussing historical, geographical and sociological trends, providing insights by relating to other expressions, etc.). On ELL, the question either lucks out and gets a good answer, or is poorly received and gets a one-line answer. On ELU, the question either lucks out and gets a good answer, or gets closed. It seems to me that ELL and ELU are on par when it comes to generating good answers, and only differ in how they react to questions that are perceived as uninteresting. (Both sites often fail to capture the interest in questions that could be much more than “should I say A or B?”.)
Matt describes the difference between ELL and ELU in different terms:
- IMO, ELU is about the research of the English Language and analysis of English usage
- IMO, ELL is really about helping people learn to communicate effectively in modern, everyday English
I don't see a dichotomy here. “Communicating effectively in modern, everyday English” is something that can be the subject of research and analysis.
Bradd Szonye express different points of view, and I think terdon's phrasing covers both answers's main sentiment: ELL is for practical advice, ELU is for pedantry. That is a dichotomy, but a false one. Good ELL answers are not weak. A good ELL answer should say: “textbooks say X, but everybody except the Queen writes Y and pronounces Z”. A bad ELU answer is equally bad on ELL. A good ELU answer may mention nuances relevant only to Shakespeare scholars — but so would an ELL answer if the asker was reading Shakespeare.
I don't often have questions about English, but when I do these days I have no idea where I should ask. Should I ask on ELL since by definition I am a learner, not a native speaker? Or is my English good enough to be accepted in the big boys' site? Should I ask on ELU since I appreciate well-researched and documented answers? Or should I on ask ELL since I would benefit from natives' intuition? I've read quite a few of these “ELL vs ELU” meta threads (though admittedly not all of them by a long shot), and I'm as puzzled as ever.
All in all, I don't see such a major difference between ELL and ELU, neither in the kinds of questions that are well-received, nor in the quality of answers that they generate.
The fact that the difference is endlessly debated and that after months of debate no consensus has been reached is a big hint that the difference doesn't really exist.