It does not matter which priority you setup F5 or Tomcat Cluster.
F5 is only for load balancing. If any new requests comes in, F5 will send the new requests to other application server depending on the setting in F5. It is not necessarily fail over to other web application server if the user is already logged in doing something, unless you cluster the Tomcat servers.
As Dennis and I, mentioned in your other posts, there is known issue with Tomcat clustering (Seemless failover) which is resolved in SP6.
Installl SP6 and check your issue. You need to cluster Tomcat for seemless failover.