It depends on who is holding the belt and what the circumstances are.
Iron Sheik needed to be a short reign champion because he needed to be the transition from Backlund to Hogan.
CM Punk was a great option for a long term reign because he's the all around best performer at the moment and it allowed for a lot of fresh matches & feuds along the way. Bobby Roode was also a great option for a long term reign for TNA because it is just easier to drive the storylines if the heel is running with the belt and you have faces to chase him.
A long title reign though can be a killer. Cena's 2007 title reign actually made me quit on Raw until that reign was over. There was no story. It was just the same thing ever month. Heel beats him up, they have the PPV match, five moves of doom, and Cena wins! Hold up belt and play music! It was pretty much just that for a year and by April 07 I had enough.