If you have redundant uplinks you'd just want to setup Load Balancing at Security Appliance>Traffic Shaping.
Flow preference are only really designed to be used if you want traffic to use one of the WAN connections over the other. A typical example of this may be your VOIP traffic. Maybe WAN1 is an ultrafast low latency pipe and WAN2 is a slower high latency pipe. Load balancing internet over both of those is fine but you may want your phone traffic to prefer WAN1. In that case you'd setup a flow preference for your VOIP traffic with destination and WAN1.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO If this was helpful Kudo me