dasilva,
The main factor is the number of users that are authenticating on the network. From there it becomes either a personal choice or a requirement depending on the system resources of the FortiGate itself and the AD servers. For example if you are only tracking 20 users, direct polling of the AD servers from the FortiGate is perfectly sufficient, whereas 20k users you would want to offload some of that work either to an agent on the AD servers themselves or its' own machine so that neither the FortiGate nor the AD servers are taxed at all.
Unfortunately there isn' t a guide that says, if you have this model and this many users use this method(i havent found one at least), but make sure youve checked out the currently available resources like below:
http://docs-legacy.fortinet.com/cb/html/index.html#page/FOS_Cookbook/Authentication/FSSO-IBP.html
https://www.youtube.com/watch?v=BfMyWBAosK0