I have seen many dyno tests with regular manifolds, stock manifolds and headers. The most you gain over regular manifolds is 15 to 20 HP. Regular and long branch manifolds seal and are trouble free. Headers are a pain to get the seal right and you have to keep tightening them to keep the seal. So, if you are trying to decide, you have to think of whether you want more or less noise compared to how much you like or dislike exhaust leaks. To decide between the two types of manifolds it's a simple question; Is 15-20 HP worth $400 to you, that's the difference in cost between the long and the short.
Jim