If there was any firm information about the release date (both of Windows 10 Mobile and devices, etc.), they will be found on the front page of Windows Central.
If it won't get delayed. With the current state of W10M there is no way W10 be as near as good as WP8.1. It's still 2 months but the speed they release new builds or even make them is just awful. They are releasing Lumia 950/XL/550 in november. I don't believe the W10M will be stable enough for it's release with new Lumias.