Headspace is one consideration when choosing a vessel for secondary. The other big factor is oxygen permeability of the material. Most plastics will, over time allow oxygen to enter through the material.
Better bottles are made of a type of plastic that allows very little oxygen penetration. Using food grade buckets would be just fine for short term secondaries (1-2months) but for longer terms I'd go with a better bottle or glass.
That being said, I don't know that anyone has legitimately done a long term study of using a different plastic to secondary in. Just the fact that you might sacrifice 5-gal of beer is enough to keep me from trying.
