The leathers are all the same except for length.
If you are an average man with an average arm length the 70K would be most suitable
If you like a little extra swing the 80K would fit the bill
If you have big hands the 90K is the way to go as you would still have a lot of length to get a full stroke in. They don't have handles or a D-ring so you hold the end with your hands.

Also, the 70K does not come with a scrub leather, only the smooth leather and a linen strop while the 80K and 90K are three piece sets and you also get a PU cover for the leather which according to
pixelfixed is important to keep dust off the leather which could affect the edge when stropping.

I use the linen to warm the blade before stropping and the scrub leather to clean and dry the blade after shaving. Works great and keeps my edges going for a long time