There all pretty much the same, the main thing you should understand is that they do not "silence" the guy the main sound comes from the gearbox however some manufactors put foam padding into the silencer to decrease of the noise of the round going through the barrel this is effective on spring rifles but not a AEG,
Most people use them as a easy way to extend their barrel length it dose have a small effect on the noise (if padded) but like Matt said 90% of the noise comes from the gearbox
As long as you get the thread right you can pick any you like the look off