Longer is supposed to mean denser, more consistant patterns. But I haven't found this to be true. They all shoot similar patterns. Personally I would buy whatever length suits what you want to do with it. Long barrels seem to balance better for things like bird hunting in open country while a short barrel is much handier in tight situations like duck hunting in a small blind.
Buy what feels right and don't worry. If it doesn't pattern the way you want you can usually try different rounds until you find one that does what you want it too.
