My rule of thumb is collagen for slim jims, pepperoni sticks etc and hog casing for all else. Sheep casings I've never used but have heard they are harder to work with (more delicate) and more expensive. Waltons makes a 30MM red collagen casing that is awesome to stuff, with a good snap and IMO also a good size for a Slim Jim. While ordering, due to S&H charges, I'd get some some of both since you will eventually find a need. There is a local store here, with some hunting & fishing stuff, where I get 22 MM collagen for pretty cheap.....so, check around, many feed stores also carry a limited supply of some items.