I prefer shorter games. Repetitive side quests bore me often, while shorter games give you something new around every corner, and keep it interesting throughout. But even for short games, a game should be atleast 10 hours long for it to be worth $60 for me.
That's part of the reason I love God Of War. It doesn't artificially lengthen the experience for the sake of providing more "hours" of gameplay, but gives you new and exciting adventure around every corner. However, there are exceptions to both, and in some games with side quests, the main campaign is good enough to keep me hooked. Also, not all short games are good either.