That is true. But Hillary is a known commodity. Her defects have been in the public eye for almost 2 decades. And there isn't much chance the Republicans have in defining her like they do Obama.
With hillary Democrats give themselves a fighting chance - a replay of 2000 and 2004. With Obama the risk is a blowout loss, and at the same time a blowout win.
I guess you can look at it as an insight to personality. Between the two I favor Hillary, and I almost always hedge my risks.