the sx-2000 is a watered down version of a Sting. Most kids simply couldn't afford the high end Sting so they got the milktoast version, and felt very lucky to not be stuck with the entry level Schwinn Scrambler/Ashtabula fork model.
I don't remember if they are welded or brazed, but they look the same except for having a straight seat post tube. The Sting was made in Schwinn's exclusive road bike shop in Chicago, which was later brought to the forefront of BMX building by Standard Byke/Rick Moliterno having them do fabrication. The badge switches to the oval stingray style instead of the round road bike badge on the Sting.
And mid-80s you have a Predator variation with Sting styling. It has the flared tubing up by the head tube, predator style dropouts. Not a memorable or desirable frameset. (I just gave one to a friend for a beater build)
an SX-2000/Sting Comp from the web:

a late 80s sting from ebay, it's been for sale quite some time:
http://cgi.ebay.com/Schwinn-Sting-Frame-Fork-Headset-Old-School-BMX_W0QQitemZ230230395783QQihZ013QQcategoryZ42310QQssPageNameZWDVWQQrdZ1QQcmdZViewItem