Almost everyone will say 3" is louder, deeper (which may not be the case).
What hasn't been said is that dual 3" catback with stock headers and cats is really not going to sound that good, and the performance benefit is infinitesimal. With proper headers it is retrograde versus 2.5" in terms of velocity due to pressure drop when volumetric flow is less than 310rwkw on a 6.0. It is worth noting you can't get near that volumetric flow without a cam or Clayton's cam (high ratio rockers), full bolt-ons and dyno tune using high octane. The main casualty is torque - exactly what your two tonne tank needs to launch. Common mistake people make is going 4-1 plus big duration which typically exacerbates the situation unless you are circuit racing your Holden.
So on a 6.0 3" is all for show, not go. 6.2 is obviously slightly different.
Problem with most exhaust videos is that the sound is poor, not same car or environment so take a look at a sound comparison with incremental changes to a 2.5" catback. Most of the presence will go unnoticed unless you turn up with headphones or system with sub BtW.
See there's quite a bit to it apart from signing up to less performance coz loud and deep all the time.
Depends what you want in terms of race pitch, idle burble, static rev-up, partial throttle (drone) issues, zero tolerance cabin noise, scare the crows or full bore all the time. Have a listen to this 2.5" burble on idle at the end - deep and horny:
[video=youtube_share;iAqNOVsxzNo]http://youtu.be/iAqNOVsxzNo[/video]
Cue to halfway in this one to realize the difference in-cabin with bi-modal drone control, also puts the slight extractor rasp on static rev-up in perspective:
[video=youtube_share;YfXNfkghTn0]http://youtu.be/YfXNfkghTn0[/video]
PS exhaust cackle / crackle / pop / fart is adjustable in your tune, not the exhaust.
