I'd recommend spending some cash and getting Red Gate's SQL Compare tool which does an excellent job, and can even compare databases to Visual Studio database projects to generate upgrade scripts. It's fast and easy to use, and works well. The upgrade scripts are also of decent quality.
It's not that expensive. Probably less expensive than your time. Just think about how much your hourly rate is, and how many hours it might take to investigate an open-source tool and get it working (and how many you have already spent), then multiply them together. That's how much a 'free' tool is really costing you, which is often significantly more than a commercial tool.