It's always good to have two or three different levels of bands, as somedays you may want ligher resistance but move your hands closer together on other days you may want harder resistance but hand further apart. It just depends on what your doing and what you want out of it. But do get 6 foot ones, as that was what was recommended when I was looking at needs to complete the HC workouts. Also you'll have a lot of choice of where to put your hands and how much resistance you want, or need, with the 6 foot.
Good luck,
Kit