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.
It depends on your market, product, and a lot of different variables. Direct mail is one of the forms of marketing that you NEED to have an expert handle for you.... unless you want to lose massive amounts of money.