onlineleben - 6:22 am on Sep 14, 2009 (gmt 0)
...I built another site that covers a slightly different version of [ * ] plus additional content...
Does this mean your sites are competing in the SERPS against each other? I would keep the old one - so you have two chances to get a visitor. Also don't forget the incoming links (already mentioned).
Maybe put a banner on pages with incoming links to direct visitors to the new and relevant content on your new site.