[MagicRat]

It depends on the rest of the combination that you are using.

Generally, relatively longer and narrower header tubes are better at increasing low and mid-range power. Shorter, fatter tubes are better at high revs - and then, only if the rest of the combination (heads, intake etc) are intended for max torque at high revs.

So, if your engine is stock or nearly stock, full length, smaller tube headers are the way to go.

FWIW, many 'shorty' headers do not produce any header-type exhaust scavenging effect. All they really are is just better-flowing exhaust manifolds.