WAN Load Balancing

Hi guys,

I have two sources of internet connection, W1 and W2 both will be configured in Fortigate 100D firewall. What could be the best configuration for W1 and W2. I am planning to configure 75% bandwidth to W1 and 25% to W2. Is it feasible? or should i leave it both 100% bandwidth for each WAN so that if one will fail, atleast the bandwidth is still 100%.

What is your suggestion guys.  Thank you.