6509 supports 2 rx/both span session and 4 tx only span session besides RSPANS
http://www.cisco.com/warp/customer/473/41.html
Difference between SPAN and capture using VACL is
--- only permit traffic can be captured, and capture can occur on egress vlan only if the traffic is L3 switched.Of course, VACL has the advantage that you can, for example, capture only traffic passing between specific IPs etc (ie, ACL based
Please post your config and someone can help out.