Best- amuse/nismo/miltek/buddyclub
Good value- Scorpion, cobra
Worst-japspeed/ebay tat
Loudest- japspeed k1, buddyclub
Good- scorpion, cobra
Quietest- miltek
Decats make a massive difference to sound also
Most of these systems are not true dual. Most true duals I've seen are aftermarket to be honest.
Somebody with more info can correct me