Trump will be the candidate. He'll smack Hillary so hard that only the dedicated will show up on both sides. Liberals tune out, independents tune out, young people tune out, poor people tune out when that happens.
Even if she squeaks by, the diminished turnout will crush Democrats downballot, meaning NOTHING will change except we aren't running backwards so much as being pushed backwards. Whoopity doo! We're still going backwards. Relentlessly backwards.
In a world where Trump is the nominee (which is looking more and more like our world), "the dedicated" will consist of every person that does not want to see a President Trump. And that's a lot of people. I wouldn't be worried.
He would only smack Sanders twice as hard. Or equally hard if you want to look at best case scenarios.
Strictly speaking on optics, I think those arguing in favor of Hillary's strengths underestimate just how damaging being part of the establishment is in this election.
In a Bernie v Trump debate, you have two populist candidates who aren't afraid to get loud and are both decently witty. It comes down to who comes off better in the exchange. It becomes more meritocratic.
In a Clinton v Trump debate, it's about the GOP having put up the candidate that couldn't be denied because the people wanted him so badly vs. the inevitable one who was forced upon the American people. That's bad optics in my opinion. It's a david v goliath story.
That said, I think a Clinton v Cruz debate favors Clinton, because that debate then becomes about policy. Clinton excels at articulating policy firmly and clearly.
In a Bernie v Cruz debate, Cruz may get the best of him. I could see Bernie getting flustered with Cruz' complete disregard for, ya know, reality and just coming across as angry in the exchange.
TL;DR: if Clinton is the nominee then Cruz is her easiest opponent. If Bernie wins Trump is his easiest opponent. In my opinion, of course. If there's polling to the contrary I'd be happy to look over it.