The 5 seater only gets more cargo space because the middle row can fold. The space where the third row seats fold away under the floor is not very useful, IMO. The main benefit is the folding bench.
That space normally occupied by the third row, is quite cavernous IMO. It is 23 x 39 inches and 7 inches deep (58 x 99 x 17 cm). Plus it is COVERED space, completely hidden from view. It was the main reason I picked the 5-seat config.
I also bought the extra sub floor ($120) that covers the pit in the back providing a storage spot for the trailer hitch, ChaDEmo adapter and other charging accessories, without consuming any floor space or consuming the frunk. (I am not sure why that sub floor is not included with the car, I find it very useful)