I use both casings, Collagen (Standard) and Animal (Pig and Sheep).
The Collagen casings are slightly tougher when eating the sausage and do not give the "crunch" when you bite into them like the Animal Intestines do. The trick to to using either is to let the stuffer do the work for you. Too much resistance when filling and either can rupture...
Humphrey's DownEast Beast W/BBQ Guru