One of Hillary's strengths is her loyalty (to her husband) , which appeals to some of the Christian right wing in America who believe in the family unit whilst her experience as a Senator for four years stands her in very good stead. This has to be seen against the fact that Obama is considered weak by some because of his lack of Senatorial experience (just two years). Although others have argued that his relative freshness gives him a clean, untainted look that is also appealing.
One thing they both must ensure is that they do not throw mud at each other in the primaries and leave any lasting wounds on the democratic challenge, because one thing is abidingly clear, and that is the need to ensure a Democratic win to try and make good the damage done to the US position in the world since George Bush turned America in to the world's biggest warmonger and destroyed the US finances in to the package.
Of course, the ideal choice might be a Clinton/Obama dream ticket. Done correctly with Hillary as President, it could ensure Democratic ascendancy for not just a two term Presidency, but a full four terms, with the Republicans in the wilderness.