What are you comfortable with? Whether you use mechanical or "by hand" the only thing to remember is be consistent. Also use the longest sander you can get for the finish work as it will turn out smoother.
I like to DA with #80 on an existing paint job. After scuffing the whole car with the DA then everything else is by hand with flexible sanding blocks of different configurations. After priming, all the sanding is by hand.
DR