Are there any pieces of research that lend credibility to one approach or another?
Not sure there is a definitive, one-size-fits-all answer, but I can give you my experience.
I've had much better success with multiple touch points than the single blast. I would specifically target folks who opened, but didn't click the first email, and send them a second email with a modified message.
If you have any more details, I can probably dive a bit deeper.