Air Purifiers

I have a 2007 Hyundai accent and Im thinking about getting either a K & N air filter or a cold air intake. Would either of them increase my horse power/gas mileage? And what one is cheaper?
also, does a cold intake require a hood scoop or other "tricking out"? Im only keeping it till i get me a truck.

Would having a very dirty air filter simply lower your gas mileage but have no noticeable performance effects on the car? Or is it possible to have that dirty air filter manifest itself in the car’s behavior in one way or another?
For example, could it cause the car to shut off, hesitate, lurch while accelerating, etc.? Or perhaps something else?