standalone vs in-router module

in-router limitations
need external nearby host w disk 
anyone have stats on flows/sec? 
effect on forwarding
point of failure in network
upgrades mean network downtime
in-router advantages 
already in box
commercial support
can do > 1 i/f 
other issues
src code control
payload sniffing 
UDP export 
benchmarking/calibration (cross-router)