Having owned the 6" version of the Skywatcher, and the 8" and the 10" GSOs, my impression is that the SW is of
slightly better quality overall than the GSO dobs (optically the 6" was outstanding, not least because of its relaxed f ratio of 8, and a great scope for the planets), but the GSO certainly aren't junk either and I actually prefer their focuser over the SW unit. My copy of the Skywatcher dob did not have a proper 2" interface, but some odd, threaded thing that was supposed to make attaching a camera easier but needed an adaptor for 2" eyepieces. My RC astrograph (a GSO) has proper 2" fittings so the SW solution seems unnecessary. My ultimate pick of the 3 was the 8" GSO, giving the best package of aperture, cost, portability and serviceability (mirror comes out for a wash every so often) and has been a great workhorse ever since I got it 3 years ago. The MDF mount is starting to show some wear but the OTA is still in great shape, despite countless trips in the back seat of my car. I sold my 10" GSO dob when moving to NZ permanently - a great scope, but a bit too great to fit on the plane with me, unlike the 8". One thing I noticed about the 10" is that the tube seems unnecessarily large at nearly 12" external diameter, leaving a big gap of unused space around the mirror. The 8" is more efficient in that respect and its tube much smaller than the 2" difference suggests.
Re 10" vs. 12" - I had the same question before buying the 10", and some of that decision making process happened
here.
Edit: duplicate post deleted