You could face an issue where you must have policy based IF the firewall on the other end requires a separate ProxyID for every ACL entry. NetScreen pioneered the idea of a ProxyID-less VPN and then using security policy and routing to control traffic. Technically according to the IPSec standards there should be a seperate " interesting traffic" type ACL and THEN a security ACL. Routing takes the place of the interesting traffic ACL.
This is a holy war/style thing that I used to argue at $previous_job all the time. I love and much prefer route based VPN' s myself and use them wherever possible. You may run into (most likely a Cisco or Checkpoint) an engineer who can' t bring up a tunnel on their side because of a " proxy ID mismatch" if so you may need to look at policy based.
-rd
2x 200D Clusters
1x 100D
1x 60D
FortiOS 5.2
FortiAP 221C
FAZ 200D