No brainer, right? The engaging speaker is of course going to keep your attention better, so you'll of course learn more. Except that may not necessarily be the case.
According to a study published in Psychonomic Bulletin and Review, well, the title sums it up nicely: "Appearances can be deceiving: instructor fluency increases perceptions of learning without increasing actual learning."
That's right. A team of psychologists had two groups of students watch a video lecture on why Calico cats are almost always female—one by a droner (called "disfluent") and the other by a more engaging, "fluent" presenter. Sure enough, the students who watched the fluent presenter said they learned more than the students who watched the disfluent one.
The kicker came when the researchers tested the two groups on how much of what they heard they had retained. Turns out there wasn't much difference between the two groups. So while a great presenter may make us feel like we're learning a lot, we're actually just thinking we're learning more.
Of course, there's an underlying assumption that being able to recite back facts from a lecture actually equals learning, which isn't an argument I'd like to make, though it passes as such during most classes I've taken, sadly...