If you use dynamic routing, you can use either the Routing Table or Round-Robin multi-WAN configuration method. Routes that use a gateway on an internal (optional or trusted) network are not affected by the multi-WAN method you select.
The Routing Table method is a good choice if:
The Routing Table method is the fastest way to load balance more than one route to the Internet. After you enable this option, the ECMP algorithm manages all connection decisions. No additional configuration is necessary on the XTM device.
Load balancing traffic to the Internet using ECMP is based on connections, not bandwidth. Routes configured statically or learned from dynamic routing are used before the ECMP algorithm. If you have Fireware XTM with a Pro upgrade, the weighted round-robin option gives you options to send more traffic through one external interface than another. At the same time, the round-robin algorithm distributes traffic to each external interface based on bandwidth, not connections. This gives you more control over how many bytes of data are sent through each ISP.
About Multi-WAN Options
Configure Routing Table
About the XTM Device Route Table