Personally i don't have a preference. Just so long as the story maintains my interest, i like both just as much. If the film is over-long (2.5 hours and above) i sometimes watch it in two parts. Can't do that when you go to the 'flicks'.
dave, I learned my lesson with those long running series when I watched Lost. I gave up long before the non-ending end (I read about it).
6 - 8 hours suits me just fine