I have just bought a new ranger 3.0 and want to increase power for towing a trailer. Which would give me more power headers or changing to a high flow muffler?

 

Headers Headers Headers!!!! Headers are usually tuned to give you more HP and Torque across the rpm range (great for towing), whereas changing out the muffler will get you a little more HP on the top end, but not a lot on the bottom end for towing. Just remember that the engine is an interconnected system, and (by the Theory of Constraints) can only flow as much air as it's most restrictive component, and on the vast majority of factory vehicles, the exhaust manifolds are one of those very restrictive components.

I would really like headers, but dont' have the cash to drop on the old Aero (a set would be a significant portion of it's value ). If you get them, either get them ceramic coated or (if you aren't worried about their warranty) wrap them in header wrap to keep the heat out of your engine bay for a little more power.