It is not that H. Clinton's proposals differ wildly from Obama's. Although we can note that she supported the war in Iraq and has not made a great a commitment to either progressive taxation or needed increased spending on social matters. The main issue in considering Clinton vs. Obama is: who will defeat the GOP? One might have imagined that H. Clinton was the better choice earlier in the race on this score, but such is no longer the case. Moreover, her artificial character as an appendage of Bill Clinton and her ties to Clinton era scandals would have severely weakened her in the general election, even has she managed to make a fitting showing during the primary season.
Now Obama himself is far from the perfect candidate. He is far too much a fan of Big Government, nor does he seem to understand what the average voter thinks about some of the more minor social issues, such as Federal regulation of gun rights. His background is also disturbing for many, esp. the Muslim ties (but this all the more reason to support him).
His church has many position that are offense to the average American, but at the same time it is a church long at the center of American political life. Obama's status as a black man (an African-American) is a major selling point, as his election would ease many racial tensions and help promote a better American culture. Most importantly, Obama has the proper ideas about the war, progressive taxation, and the need to shift Federal spending from militarism to support for civilization.
[Addendum 4/12/07: It no longer seems to me that either candidate can beat McCain. McCain has the power of Zionism and the military-industrial complex behind him, and (vis-a-vis Obama) the power of pro-Anglo-Saxon sentiment, while Hillary Clinton and Obama are rather weak candidates. As such, at this point in the race, "Who can beat McCain?" is no longer the central question.