Here's the deal, everything you are ever going to make has a purpose. That purpose is what you use to decide materials of construction, leather types dyes, rivets, threads etc.
You will get a dozen answers and about the same amount of arguments on what material to use. Suede has its uses as does everything else. The holster debate will go on forever, personally I don't line holsters unless its wanted its a waste IMO guns have no feelings, they don't feel smooth or rough texture and keeping a gun/holster clean has more to do with the owner than the leather but that's not the question here your making collars right? i feel that same about phone holsters too, line em or not it doesn't matter the phone doesn't care, only the guy giving you the money cares.
I think there is some confusion in discussions here as each crafter has his own methods depending on how he sells. I sell, as do others, to customers based on their wishes. Some sell in bulk , make a dozen of something and sell it at fairs online etc. Some are hobbyists making literally anything they want or need, some are making a living at this or at least attempting to and sell preconceived products, all exactly the same. All these things dictate to some degree what you use and why you use it. What a person likes against their skin is purely a matter of personal opinion so making a blanket statement that this leather or that is the best is just silly. I suggested suede for a dog collar but you can sure as heck line a collar or anything for that matter with any leather you want or not its up to you.
So with this rambling on of mine I'm trying to get to a point i think. What you really need to confirm is what you are making. why you are making it. how you are going to promote it. and how you are going to sell it. That will decide, to a point, what materials you will need and why you will use them. If you like suede lined collars make em, if you like smooth lined collars make em, make both and see which sells best to whom ever your intended market is.