I'm in a similar situation, except I am/was far less impressed w/ the Captivate. It's buggy, laggy, and the battery life is barely tollerable. Aditionally the build quality is not impressive especially the power cord port, speaker and audio jack.
Admitadly it was great for the first few months, but after AT&T rolled out an update the phone turned into a hunk of junk.
Unless the SG2 is a 100% improvement I wouldn't personally consider it.