Depends on your goals.
More reps at a lighter weight works endurance (how LONG can you lift), while fewer reps at a heavier weight works strength (how MUCH can you lift) as well as causing more muscle growth.
Whatever system you choose (and it's good to mix them up: doing some strength and some endurance), ideally the last 2-3 reps of a set should be difficult to do (but still possible with good form), and you should not be able to do more reps with good form. That is the most effective way to train.
The reason you can lift more with a barbell is that there are fewer stabilizer muscles that have to work to lift a barbell, and the strong side can help the week side out. Good reasons to not JUST do barbell work, but to include dumbbell work.
As for which video to get. What level are you at? What kind of workouts do you do already? Are you looking for cardio? strength? a combo?