It depends on what you plan to put on them. I bought 70" Yakama Core bars and cut them down(I think 66") to the size required for two Thule Hullavators as the Hullavators need to be a specific distance from the sides of the car.
I went with the Core bar since it I knew I could put it down. Because of the way the aerobars mount (slotted underneath) I am not sure if or how much they can be cut down. The Core bars are steel just like the round bars.