Understand, a muscle contracts the way it contracts, PERIOD. Think it through. It'll become very clear.
E.g., a muscle brings your arm towards your body. ANOTHER muscle rotates that arm. How you effect that contraction (which exercise) really doesn't matter if the exercise allows a full range of motion and a stretch.
Dumbbells allow a fuller range of motion, require stabilizers, and allow a full stretch. Barbells often involve more muscles in the body / less isolation, but, often can't be done through a full range of motion or folks don't do them right.
You'll want to study up on ATP recovery for the answer to your last question. At your level of training, there's very little difference, but, because of ATP recovery in the alternating method you could go a bit longer in an ideal situation because of the recovery between reps, if rep count is the metric you're interested in. If "pump" / engorgement is the metric you're interested in then sets to failure, one arm, then, the other arm, would work slightly better. You just needed to think on it some. You'll want to try some of this yourself to get a feel for it.