The Flex 3401 is a beast!!
IMO the Hybrid Pads from Lake Country are the way to go. I believe the Pads were designed for the Flex 3401. The Orange Pad is the heavier cut for the Hybrids. I cannot comment on the LC Yellow Flats as I have never used them as I had excellent results from the Orange Hybrid Pads whether it be the 5" or 6.5" Pads.
I use the 6.5" Pad more than the 5" Pads. However, the 5" Pads give you better control and more of a "finesse" pad. The 6.5" Pads cover more area.
The Flex 3401 is the closest machine to be a rotary. The forced rotational oscillation is 2nd to none. It can work on curved panels and can tackle any correcting needed when compounding/polishing. I also use it to apply sealant/wax too.
Bottom line the Hybrids are the way to go Orange, White, Black, Red or Gold Pads.