At this point Sigil is no longer being actively developed. Moving development to Github has netted a few contributions but they were one offs and fairly minor. With Sigil development being stalled, Kovid (of calibre) starting making the tweak epub functionality in calibre into a full editor.
calibre's editor at this point is stable and has many of the features, though not all (yet), that are present in Sigil. Like Sigil, calibre's editor is open source and unlike Sigil is being actively developed. I've known Kovid for quite some time (though calibre) and I'm confident that the calibre editor is the way forward.
For people using Sigil, keep using it as long as it works for you. If you find it's not meeting your needs or if you want to see what else is out there I recommend checking out cailbre's editor. While it doesn't use any of Sigil's code I consider it Sigil's spiritual successor.