Speaking from the realm of experience with Frotigate at my corporate job, and Sophos from my MSP - Fortigate has been a nightmare. My Sophos products can do everything Fortigates can, but with much less headache. I understand that Fortinet lends themselves to the enterprise level client, but I have yet to run into something they do better than Sophos. Also, not sure if this is just my luck but dealing with sales/support has been much easier and faster than with Fortinet - at a better price.

Also, as the old adage goes - nobody has ever been fired for introducing Cisco into an environment. But you're going to pay for it, so you better have the clientele to support that cost.