My gut tells me that Trump would actually do much better against Hillary Clinton than Bernie Sanders, in large part because the only Democrats who seem really fired up right now are Sanders supporters (and this is taking into account the anti-Donald vote that would be present in a general election). Not predicting Trump would win, I just think he provides Clinton with more "matchup problems" (especially enthusiasm and on trade issues) than he does Sanders.
And btw, my gut rarely lies. Unless there are pies that have been on the counter for a few days.
Agreed. Trump is a matchup problem for Clinton, and not just for the reasons you mentioned, i.e. trade. Trump can go all out populist against Clinton, for example, he can target Clinton's votes on bank bailouts/TARP, connections to Wall St, and so on. There are other issues where he can maneuver to her left as well, such as Iraq, and interestingly- health care. Clinton lobbied for Obamacare, which gives Trump an opening (should he choose it), to advocate for something like single payer, which he's supported before. I'm not sure exactly what route he will take, but if he goes in this direction, Trump would arguably be the more liberal candidate on the ballot, the first time for the GOP since '76 (before then, the GOP was regularly the more liberal party).
Of course, none of this would work against Sanders. Trump would have to rely almost entirely on style/optics in that election, since Trump/Sanders, outside of rhetoric (e.g. comments on Muslims etc), are actually probably more similar on issues than Trump/Clinton. Trump would have to win by doing things like calling Sanders a socialist repeatedly and making him look like a crazy old man (both lines of attack actually could be fairly effective).
Along those lines, against Clinton, Trump is the only one that could effectively go populist. I don't see Cruz doing that. Maybe Kasich a little bit.