One important note: PBR does not apply to traffic from the firewall itself. Traffic the firewall sends (e.g, connecting out to Check Point for IPS/AV/URL Filtering data) would take only the normal routing table, so would only take one of the ISPs.
Another option might be VSX. It allows you to set up multiple routing tables on the firewall (like Cisco VRF, Fortinet vdom, Palo Alto sys). Traffic routed to one firewall interface hits one routing table. Traffic routed to a different interface hits a different routing table. Each firewall context would then handle a single ISP.
