Trade-offs between lower back exercises?

Referring to http://www.chap.com/lback.htm, what are the pro's and cons between:

1. Dumbbell Good Mornings
2. Dumbbell Deadlift
3. Barbell Deadlift
4. Barbell Deadlift From Bench
5. Strightlegged Barbell Deadlift (not shown but similar to #4 without the bench)

Furthermore, when doing #5, someone showed me how the bar is held with one hand
facing forward and the other hand facing backward. Why is that?

Thanks!