Dual beater mixers tend to leave stagnant batter at their periphery, recycling the same stuff over and over between the beaters but leaving an ample amount outside. Offset beaters and a rotating bowl helps a bit, but there still seems to be an uneven distribution of mixing action.
The planetary action is more thorough - whether it's for cake batter, cookies, bread, or whipping egg whites.
Just my non-expert opinion, mind you.
What the dual beater mixers have going for them is that they are generally less powerful, lighter weight, and hence easier to store and retrieve and operate than a big planetary model.
And either technology beats mixing things with a spoon by hand.