IMHO, there's no noticeable difference for the average Focus owner.
Any gains/losses will be seen with more and more power and RPM. I'd almost go so far as to say that below 7,000RPM and/or 160whp you won't see any difference.
Every long tube header is an improvement over stock. Other than that it's build quality, durability, fit/finish and personal preference.
121,000 members and still no sponsored racing.