Watching tonight's debate, I can't see that there's any difference. Clinton seems to concede that there won't be any enforcement for her health care "mandate." As Obama has noted elsewhere, unless there's enforcement, the "mandate" is just a gimmick and political talking point. People who don't want to sign up for a health care program--subsidized or not--won't do so under either Clinton's or Obama's plan. Timothy Noah in Slate has a good discussion of why Obama's on the right side of the mandate issue.
UPDATE: The Washington Post's Fact Checker claims that the estimate that 15 million would not be covered under Obama's health care plan comes from an extrapolation from the Massachusetts plan. As far as I know, that's incorrect. According to the Washington Post itself, and the two other fact check sites, the "dubious" figure comes from a rough guesstimate in a New Republic article. From the Obama website:
"Clinton Uses A Dubious Statistic When She Claims Obama's Plan Would Leave Out 15 Million Of The Uninsured." FactCheck.org reported, "Clinton uses a dubious statistic when she claims Obama's plan would leave out 15 million of the uninsured…Clinton based her claim on a column by The New Republic's Jonathan Cohn, who loosely estimated Obama's plan would leave 15 million uninsured…Cohn makes it clear here that he is offering an estimate based on the best information available, not a hard and fast calculation. And the best available information doesn't always agree." [FactCheck.org, 11/16/07]
Clinton's Claim "Based On Too Many Hypotheticals To Rate More Than A Half True." "It's a tough call, but because of the disagreements here, we find her claim to be based on too many hypotheticals to rate more than a Half True." [Politifact, 11/15/07]
Clinton Cited "Hardly An Authoritative Source" For The 15 Million Claim. "So where did Clinton get her figure of 15 million uninsured under the Obama plan? Her website cites an article in the New Republic, hardly an authoritative source." [Washington Post, 11/15/07]