If you use SSL VPN, it has to be FortiClient. But if IPSec VPN, technically clients can be anything as long as you can set proper parameters for phase1 and pase2 at the client. FortiClient, as you know, of course can handle both (selection button at the top). And it's easier to open a ticket with Fortinet TAC in case any problems because they know both ends. I personally haven't used any other clients but FC.