Usually the state link should be as fast as the fastest interface.
http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/failover.html#wp1051759
Failover Interface Speed for Stateful Links
If you use the failover link as the Stateful Failover link, you should use the fastest Ethernet interface available. If you experience performance problems on that interface, consider dedicating a separate interface for the Stateful Failover interface.
Etherchannel option is not available.
-KS