I use natural Hog Casings for bratwurst and emulsified sausage (frankfurters) I prefer to get them liquid-pack, right from my butcher. I do enough business with him that five-or-ten feet of casing is often gratis when I pick up my pork or beef order. Barring that, I can buy 50 feet of salt pack for about $6 from him. (I like to keep them around in the summertime for random acts of brats.)
Naturals need rinsed (and soaked if saltpack) and carefully watched for holes. They can be twisted and snipped, have that nice curve, and nothing *bites* like natural. If I ever served my 90 year old Polish Dzia-Dzia his kielbasa in anything but naturals, he would disown me.
I use fibrous casings for my Summer Sausage, get them from Cabela's for about $20 (to do 50 lb of sausage) They need to be soaked for at least 30 minutes and come tied on one end, the other end must be closed, either with string, hog rings or zip ties. They are not edible and need to be peeled.
Collagens are NOT soaked, used dry. Cannot be twisted, must be snipped (risk losing filling) or tied (time consuming). Easy and cheap for brekkie sausage and as they need no soaking and have no special storage requirements, are handy to keep on hand for when you are big on meat and low on time. Sheep casings (the skinny naturals) are too spendy for me to use on something as basic as brekkie sausage.
Feel free to click some of my sig line links to see casings in action! ;) LOL
Cheers and LUCK!!