I do tend to take the idea that a bunch of Republicans won't vote for Trump more seriously than the idea that a bunch of Democrats won't vote for Clinton. Bernie or Bust sentiment is largely being fueled by disappointment at one's preferred candidate losing the primary. The ones who actually go through with it are most likely people who typically don't vote or go third party anyway.
Republicans I think are more likely to follow through on not voting for Trump because their reasons for not doing so are based more on problems with the candidate himself. They see him as harmful to the party itself (true, though they need to recognize he's a symptom of deeper problems) and also potentially destructive to the economy given his stances and unpredictability. There's a reason a bunch of party insiders are openly plotting to steal the nomination from him and why they're willing to help out Ted Cruz, a man they almost universally loathe, to do so.
At the end of the day most Republicans will fall in line behind Trump if he's the nominee, but I still think you'll see an unusually high number of them defect/vote third party/only vote downballot/stay home this election, much like happened with Goldwater in 1964 or McGovern in 1972.
