A blowoff valve vents to atmosphere, generally with an audible sound. You hear this as a "chhh" or a "whoosh" when you shift, or otherwise let off the throttle while building boost.
A bypass valve does the same thing, but as others have said, it recirculates the air so for the most part, you won't hear it.
Many of the modern valves can be both a BOV (vent to atmosphere) and Bypass (recirculate) Sometimes you need an adapter to convert the BOV to Bypass. There are many valves out there, and this will depend which one you choose.
The stock valve is the bypass type. It's not a very good one. It's also ~20+ years old at this point. (it has a rubber diaphragm inside)
Note: The stock bypass valve does not have a strong enough spring to be used as a BOV.
With our very small turbos, the performance is not really noticeable, but you can choose too large a BOV (not sensitive enough) or adjust it wrong (opens at too high a pressure) and damage the turbo in the long-run. Look up "Compressor Stall" for more info.
What it comes down to is mostly personal preference:
- Are you exceeding the limits of the stock bypass valve? (extra boost, larger turbo)
- Do you want the audible air discharge unique to turbo cars?
- Are you capable of doing minor/major fab work to install the valve?
Common BOVs
-HKS SSQV
-Greddy Type RS
-no-name piston style w/ 19mm hose barb, nearly direct fit to the factory piping
If you choose to vent your BOV you must block to return pipe