depends on what you are wanting to hear....I don't have any knowledge on the subject, but personally I want the quietest without affecting performance. Others may want the loudest or anywhere inbetween...
I guess it depends on what you want them to sound like. Mellow or loud? In my opinion, you don't want it sounding like your grampa's Honda. I like a nice, crisp, nasty bark with plenty of back pressure when you roll off the throttle and then fairly quiet and purring at idle. I'm running bone-stock peashooters on my '73. Here's an old post I remember reading:
I wouldn't view that as a negative, Cookie. It's just one of the characteristics you'd expect from an excellent long-distance cruiser. Lot easier on the eardrums after a long day of riding. I just wouldn't like it on my Norton is all!
I have a set of peashooters from Rgm that don't appear to have much inside...when you look through them you see straight through, but there is perforated walls where the pipe flares. They sound great..dunno if they are 'long' or 'short'...It's an 850 so I reckon long.
I also have a 650 Triumph with sweptback downpipes and 'Goldie' Silencers (long). They sound fantastic and have a similar construction.
You havn't bought a British bike to sound quiet...cant understand that concept...so I say get 'straight through' pipes.
The other thing to remember is this...
Long pipes mean deep booming notes, short pipes mean agressive raspy growls. Which do you preffer? Also remember that an engines state of tune is affected by the length of the pipe, so sticking to a similar length to original may men less messing around after with carb settings etc.
The only peashooters I've had that were awful sounding were made by Domi Racer. What a POS. Very restrictive too, I lost noticeable power. My current silencers are smooth weld Wassel and they're lovely.