Well, in a nutshell, one type is longer than the other! Both types of headers will improve your engine's performance over a stock exhaust manifold because they offer less impedance to the exhaust gasses as they leave the cylinders, and because they create a scavenging effect, thus allowing the cylinders to fill more completly with a fresh charge from the intake.
Generally speaking full length or long headers will make siginficantly more power than their shorter counterparts. Shorty headers however are easier to install and as far as I can tell are the only type of headers aproved by the EPA for use on late model cars. High end race motors which are tuned to make maximum power in a narrow rpm band usually get headers that are all of the exact same length that are tuned for that specific narrow power band, but on street motors this isn't necessary.
If space is a concern look at the old fashioned try y headers as a possible solution.
One important note: headers will not make a significant difference in performance if the rest of the exhaust behind them is imposing major restrictions on the exhaust flow. For them to work properly you need a good dual exhaust system with good mufflers, preferably a crossover pipe, etc.
Unlike most engine mods you can do short of enlarging the engine's displacement or droping in a bigger motor headers will significantly improve bottom end torque as well as horsepower.
Thats all I can think of right now, I am by no means an expert here so somebody please correct me if I got something wrong or missed anything, hope this helps.
<P ID="edit"><FONT SIZE=-1>Edited by curiousgeorge on 03/23/01 11:27 PM (server time).</FONT></P>