Although there are a number of differences between the two sites, the key difference is right there in the name:
English Language Learners (ELL) is about helping people learn English
English Language and Usage (ELU) is really about the study of English Language and Usage from a largely academic perspective.
If your question is really about how to speak, read or write everyday English, it probably belongs on ELL. If your question is about historically why things happen they way they do in English, or about how to speak, read or write specific uncommon or obsolete dialects of English, it probably belongs on ELU.