The Flex has enough power that it will rotate and oscillate any pad you place on the backing plate.
So the choice comes down to personal preference.
Open Cell foam will last longer, that would mean getting either 6.5" CCS or Flat Pads.
Closed Cell foam offers less tensile strength and will wear faster but many love how they perform.
The safe choice would be get one of the open cell varieties. Open cell also washed and dries better and faster than closed cell.
As for your actual pad choices, there is a great variety of levels of aggressiveness or non-aggressiveness in the CCS line as there's only 4 options in the flat pad selection.
Here's the link to our Lake Country Buffing Pad page where you can find all your pad options for the Flex 3401 including the new Hybrid and the PFW and BFW-Hybrid pads. Lake Country Buffing Pads