I've been wondering the same thing. Don't know that I have an answer but found this in a write up on catch cans on ebay - course I have no idea how authoritative the writer is...
Breather style Vs. Sealed
Not all cars have been designed the same. Weather high performance, daily driver, off road, motorcycle, or combination, there is a PROPER catch can to benefit your vehicle. How do you know what type is best? First you have to determine what the vehicle will mainly be used for.
Is your vehicle high performance? Do you race or drive it like you stole it? Then you would probably benefit more from a Breather Style. This style vents the blow-by gasses to the atmosphere while catching the sludge in its resivoir. It may not be legal to run this type in some states for emissions control purposes. Allowing the crankcase to vent freely will prevent unwanted drag on the internal engine components. This frees up the pressure being created and all the other benefits of the sludge being separated from the fuel. Honda, Nissan and the like are most common to use a breather style.
Is your vehicle a daily driver? Are you trying to get the best fuel mileage possible? Then a sealed type may be best for you. This type of catch tank allows your blow-by gasses to be re-burnt while separating the sludge. Some PCV type systems will require the metered air to re-enter the motor in order to run properly. This is from the way the vehicle was designed. Most high tech vehicles are this way. For example BMW, Volvo, and so on. This sealed type is best for passing emission tests and will not adversely affect the test outcome. It may also HELP the test outcome by filtering out the bad stuff and allowing your motor to burn the fuel more cleanly.
Here's the link to the entire article...
http://reviews.ebay.com/Buying-the-RIGHT-catch-can-for-your-vehicle?ugid=10000000019384879
