Can anyone tell me the real difference between these pads for the Flex 3401?

Both are sold by PBMG at AG and Autopia.

I guess I am kind of lost between all of these different pad offerings.

Is there a preference between the 2 of them for use with the 3401?

Which backplate do I need to be able to run these on the FLEX 3401?

My 3401 came with the standard 5.5" backingplate and also a 4" plate.