El Capitan brought on some very serious bugs for me and many people. Namely the entire system completely freezing when watching YouTube videos (this has been fixed in 10.11.6) and Mail not working (hasn't been fixed). So I've almost been a year with an OS that is constantly freezing and not working. If I had skipped El Capitan altogether, I would have had much better luck for a whole year.
This time, I'd like to skip Sierra altogether for this reason, but since Mail is still not working, I'm hoping that Sierra actually fixes that. If it does fix Mail and doesn't introduce any serious bugs, then I'm sticking with Sierra and skipping whatever OS comes after it. But to find out, I need other people to act as guinea pigs and upgrade before I do, then post on these forums about any bugs. Usually it takes about a month for some issues to show up.
I would say that an Apple OS still only matures after about the .5 release, at which point it can be considered obsolete anyway. So really, the best time to upgrade is a year after release. By that time, all bugs will have been reported extensively so you will know everything in advance.
If your system works fine on El Capitan, then I'd recommend you skip Sierra and not takes unnecessary risks. If El Capitan has issues for you, then wait 6 months after release and then upgrade, in hopes of having less, and not more bugs. Ideally, wait a whole year and upgrade to Sierra when the next release comes out.