OK, here's the skinny.
Recirc
Pro's
-No super rich condition with a MAFS between shifts that can cause hesitation
-More pressure remains in the hotpipe between shifts allowing for faster response between shifts
Con's
-Will often need a much stronger spring than a atmospheric valve due to the vacuum from the intake of the turbo acting on the valve to try and open it in boost. This can cause issues in high boost situations where putting a strong enough spring to keep the valve closed in boost will not allow the valve to open at low boost levels and you can get low boost compressor surge (turkey). This isn't an issue with the Synchronic and HKS valves due to their design.
-Not as much sound
Vent
Pro's
-Sound
-In high boost situations with out a MAFS, the valve can be "tuned" to work over a wider range of boost pressures with a lighter spring.
Con's
-In MAFS systems it may cause issues with the idle/running between shifts or after letting the throttle off due to metered air being vented, causing a very rich condition.
-Not as responsive between shifts as a recirculated setup