Hi
Every cisco switch has SPAN by default.
The choice will basically be dependent on following critiron:
(1)No of users
(2)Redundancy requirement/Uplinks
For SMBs Cisco 3560G and 3750G is an idle solution.
For Large Enterprise, Above switches at access layer along with 4500/6500 for distribution is an idle choice.
http://www.cisco.com/warp/public/473/41.html
Regards
JD