The base section that mounts to the tripod is the same for 5.5', 8' and 12' cranes and the second half is different length to make the the crane 5.5, 8' or 12'.
The 8' for can get to close to 9' with the tripod head mid chest. The short tip or the 8' interchange very quikly so you can work in tight quarters, Th 12' is a serious crane and is useful in large spaces like auditoriums.
I would get the 8' and purchase the short extension later if you find the need for a more compact crane. As I said the 12' is a monster and most shoots won't rquire that size but you can extend the 8' with the purchase of the long extension.
I am extremely happy with my Kessler crane. I bought the 12' version and added the short tip. Very versatile for many shots. You will probably wind up getting all the attachements, so I would just buy it outright and be very happy with it.