In traditional level-based systems ala D&D, a game that runs for five to ten levels is ideal, and ten is still kinda pushing it. So I prefer shorter campaigns, for a few very important reasons:
1. Life is busy and hectic, for me and for my friends. We just don't have time for slow paced plots and long drawn out stories anymore. Back in high school we'd have entire sessions devoted to "filler" episodes (shopping in the market, etc), but nowadays we start where the excitement does, and the "boring" stuff is handled between sessions.
2. There are so many different RPGs to play out there, and not enough time to play them all. By sticking to "shorter" games, we get to play with more systems, having different adventures, telling other types of stories. Having said that, I don't usually like one-shots, because they don't really give me enough time to explore and develop whatever character I'm playing, or truly embrace the mechanics of the system.
3. Despite constantly romanticizing the "level 1 to 20 campaign", the closest I've ever gotten was level 12. If we want more power characters now, we'll just start at a higher advancement (whatever that means for the system we're playing).