The polls I've seen show Trump doing better against Clinton than Sanders. Said another way, people seem to be willing to vote for Sanders over Trump to a greater extent than they are to vote for Clinton over Trump. So in terms of who can win the general, it seems the better choice is Sanders.I've heard this several times. I'm skeptical.
I think, quite simply, that the GOP has been so hung up on its internal battles that it hasn't turned its attention to Bernie yet. But when it does, I fully expect the full extent of the GOP's "turn the Dem candidate into an America-hating demon" forces against him. It's possible Sanders could still win the election — Obama and Bill Clinton both survived the process and won the presidency. But I'm not sure he'll be much more electable once Republicans decide to target him in earnest.
Sanders could possibly beat Trump more easily than Clinton could today; will that still be the case in November? A lot rides on the answer to that question.