Speaking as a Unix & Linux moderator, any question about Ubuntu Linux would also be on topic on U&L. The scope of Ask Ubuntu is almost subset of the scope of U&L. The exceptions are things like the Ubuntu phone and Canonical services like Launchpad. That said, the folks on U&L tend to have little interest in GUI stuff and the vast majority don't use Ubuntu and certainly don't use Unity (a desktop environment which is not very popular outside the Ubuntu world).
So, while you can post your Ubuntu questions on either site, if the question is about Unity or, more generally, if you really want a GUI answer, you are more likely to get an answer that meets your needs here.
To give an example, if you ask about configuring a network interface, on U&L you will probably be told to edit text files while on AU you will most likely be given screenshots of the various GUI dialogues you would need to go through. Both answers would be correct, both would actually do exactly the same thing, but which one you prefer is up to you.
So, in conclusion, the only "best practice" is i) never post non-Ubuntu questions here and ii) post Ubuntu related questions on whichever of the two sites you prefer.