Just for clarification, there are two possible redundancy modes, one is SSO (it's the better generally) and the other is two standalone WLC in a mobility group and with specific configuration on the APs (they need to have both WLC configured in their high availability tab).
For SSO, just make a redundancy failover, all APs will move to the second WLC without any traffic interruption (normally, or an extremely short one). In this case the previous WLC will reboot and then be the redundant secondary. Downside of SSO, if you do a software upgrade, both reboot at the same time, including all APs.
For two WLC (this is N+1), if you reboot the "primary" WLC, the APs will move to the second after a short time, this will cause a short traffic interruption. Advantage here is, you can upgrade the "secondary" to a new software release and then join some APs to the secondary to test the stability. If APs of the same building are split between the two WLC, then this can cause roaming issues/delays.
[edit]
I forgot one big downside of the N+1 variant. With N+1 both WLC require enough licenses to handle all APs if the other WLC is rebooting/broken. With SSO, only the primary needs AP licenses, the secondary will inherit them.
