Honestly, I think Trump is the worst possible option for the Democrats to face. A Democratic Presidential victory in 2016 is close to guaranteed barring some absolutely shocking sequence of events; 2016 isn't about the President, it's about the Senate and the House. American politics is hugely partisan, and swing voters comprise a tiny minority of the electorate - the ANES finds that only 13% of voters are genuine swing votes; everyone else has made their mind up for the party they want regardless of essentially anything else. Since 1972, there has only been one election where swing voters were responsible for a presidential win (more precisely, where the majority among swing voters overturned the majority among non-swing voters), and that was Carter's '76 win over Ford. In every other election, you could remove all swing voter votes from the result and still get the same Presidential candidate (although admittedly with different margins). Why? Because what matters isn't swing voters - it's turnout rates amongst people who identify with your party.
Almost uniquely in Western democracies, in American politics, you win by firing up your base - that is, you don't try and get people who are going to vote but might not vote you, you try and get people who are going to vote you but might not vote. And hell, Trump is good at that. Trump's really good at that. Look at the demographic break-downs for whose support he gets - he's getting older, whiter men with little to no education and low-incomes from working class backgrounds and blue collar jobs, compared to e.g. Rubio who is sweeping the wealthy, educated Republicans. But Trump's demographic is normally a high apathy demographic. They don't normally have this kind of turn-out, and they don't normally exert this much influence on the Republican primaries (obviously, as we've never had a serious Trump-like before). This is normally a base that is ignored by frankly everyone, and has never really had any kind of serious political representation before.
That's a hell of a dangerous demographic to be going up against. It's like the Republican version of Obama's young and black coalition - a political reserve that had never quite been tapped in that way before. If Clinton or Sanders end up facing Rubio, the national result will be much better for Democrats than if they face Trump. If they face Trump, they get a few more swing voters. If they face Rubio, then they end up facing an apathetic Republican electorate that won't turn up at the polling booths in the same way they would for Trump. Trump will keep the Senate close and the House out of Democratic hands, whereas Rubio or (best possible plausible option) Jeb would give the Democrats a very real chance at the House.