I voted single, multi-session adventure because that fits with how I'm gaming now (i.e. school and work combined take up too much time to make my own adventures like I used to), but if I were making my own adventures again I'd pick up just a side-trek or two to have on standby in case I needed another week to finish what I was working on.
My ultimate preference however, would just be a book of dungeon maps. Mapping out the dungeon(s) used in an adventure is the most time-consuming part of making an adventure for me.