If I use Ubuntu 15.10 and I have a question, how would I know to post it on Ask Ubuntu or Super User? Does it make a difference?
It is completely up to you which site you ask on.
Ubuntu is perfectly on-topic here and there is a good chance that you will get an answer here, but your question is also likely to get an answer on Ask Ubuntu as well.
Due to the overlap in scopes the main question you have to ask is which community you think might be able to help you more. Super User has a more general population of users from all major desktop OSes, but Ask Ubuntu has people who are focused a lot more on Ubuntu specifically. If you prefer the community on one site over another then that can be a deciding factor in many cases as well.
We will not simply migrate on-topic questions from Super User to Ask Ubuntu or similar sites that have scope overlap without input from you, so it really is up to you where you ask it and it is even possible for you to ask us to migrate it to Ask Ubuntu if you do not feel your question is getting the attention it deserves. See Migrating questions to the Ubuntu or Linux/Unix Stack Exchange sites
It depends on the issue. In many cases what you might feel is an Ubuntu-specific issue is often just a general Linux issue that could be handled here by someone familiar enough with Linux/Unix to understand that. So Super User would be a decent fit.
That said, if you have a question that is definitely focused on unique Ubuntu things, then Ask Ubuntu.