Re: Hand or machine...
As with almost everything in detailing, the answer depends on a number of things.
If you want the flattest finish possible, try sanding by hand. There are also times when an interface pad just isn't going to contour to the surface like a hand pad and sheet of paper.
I really like DA sanding. It's not as strenuous as hand sanding and the results are very close. The latest DA disks are getting better and better at leveling the surface. The time it takes to DA sand a car is much less, too.
Both types of sanding will have their drawbacks. DA disks can produce tracers, while hand sanding can produce tracers.
Sanding is one of those skills that can not be acquired over night. With enough experience, one can learn how to minimize the drawbacks, while producing a better finish.