I visit Super User now and again, and I get amazed by the amount of Unix & Linux Questions asked there. Seems like many don't know of the existence of this site (or they simply resist?). This worries me because we may be missing some juicy Q&A, and that leaves one to decide which of the sites to focus on. This is sub-optimal, so I'd like to hear other's thoughts?
My rule of thumb recently has been, ask here first. If I don't get a good response then cross post to SuperUser or ServerFault. I understand that by doing this it might take me a longer time to get an answer, but doing so helps Unix.SE grow and keeps it full of fresh content. I feel that's worth taking a possible hit on visibility so to speak.
In response to the question "The real question is why Unix.SE is a separate site!" which Gilles posed, I counter that far too many people avoid SuperUser because they feel it is a "dumping ground" for questions from StackOverflow or ServerFault which are not programming / sysadmin related. Many people I respect have told me that they flat-out ignore SuperUser. As Unix.SE has grown it has avoided such preconceptions.
I just had a question closed on SuperUser for posting it here. I talked to the mod responsible (slhck) and he pointed me to a discussion on meta.SO that apparently states the official Stack Exchange position.
To make a long story short, cross posting is frowned upon.
Unix questions are on-topic on Super User. This site is in beta, which will put some people off. This site is a lot younger so has a lot less visibility — many people will find SU from word of mouth or Google searches, and won't have heard of Unix.SE. So why wouldn't people ask unix questions on SU?
The real question is why Unix.SE is a separate site!