Personally, I strongly prefer custom campaigns, because I feel like I have more freedom as a player. Even when they write in alternatives, you are still on rails. There are just junctions on the rails. If you do something unexpected, the pre-made campaign falls apart. The custom campaign is easier for the DM to adapt, because it is their creation and they intimately know all the moving parts and how things will react to what you do. A decision in the game can have huge, long term effects that a pre-written campaaign won't put in because it radically alters how everything works. For instance, a player unexpectedly managing to f forge a deep freindship with a powerful faction, and gain their support. Something like that can take the campaign in a completely unexpected direction. A prewritten campaign will try to resolve your decisions as fast as possible so as not to bifurcate the possible storyline. Since they have to write every possible branch, this is undesirable. In a custom campaign, you only have to worry about what choices actually happen, so that issue is nonexistant. If you do let those changes happen in a prewritten campaign, then you end up on a custom campaign based on the prewritten one.