the forge is a stock oem metal replacement more or less, it makes whatever sounds the stock bpv makes, but like coy said, instead of plastic snaping together, you hear a metal 'sword clashing' sound. it is overwhelming inside the car sometimes. it is high quality, and is tunable with many different mods.
the hks ssq is a high quality and respected brand peice. it is very up to date on build and technological advancement in turbocharging, BUT it sounds the absolute riciest of the bunch because it is a unniversal build attached to a MS6 bolt on kit so it goes on Eclipses and Civics just the same with a different bolt on kit. swooooosh tweet, swoooosh tweet, or in recirc you get a sped up turkey call.
turbosmart is the highest quality, most expensive, and most designed specifically for the MS6. although it is also a universal build with a bolt on kit attached, it comes in large size if you dont mind making small cuts to the intercooler shroud (worked fine for some) or you have a FMIC, or it comes in compact size and fits perfectly (no difference in performance). It is a half and half BOV and BPV, It sounds the most normal and indescript of all the choices, you hear a crisp metal "PASH" much like a commercial truck. it is also very easily tunable. and its pretty bulletproof.
these are the top 3 choices i'd say. anything else you may be taking risks. even these, you are taking risks. the stock bpv works, and once you have an intake, you can hear it very well, and it is not ILLEGAL.
but thats just me.