You're right that Hillary & Rudy have similar positions on social issues. However Rudy has an actual track record of achievement in cleaning up NYC, plus he's an authentic New Yorker instead of some carpetbagger from wherever. I think he could defeat her 2006 senate reelection bid and pretty much take her out of contention as a 2008 presidential contender.
I really wish he would do it since he has about zero chance of winning the Republican nomination in 2008.
Not just on social issues.