While there's no doubt that the Vortex is slightly better than the Phantom at reducing muzzle flash, it also has some liabilities, particularly when you have to permanently attach it.
First and formost, a number of people have received Vortex's from a batch that received an improper heat-treat. These ended up having their tines bend and break off, leaving a permanently-attached device that didn't suppress flash nor meet NFA length requirements. That's bad. The Phantom, being closed in front, doesn't have that problem.
Also, the Vortex gets caught on brush easily, which also puts stress on the tines, and can impead forward progress of the rifle's owner. Last, the Phantom, like some Russian and German guns, has slots on the front that will allow you to place the muzzle on a fence wire and fire, quickly cutting the fence. (Granted, this is probably only useful on a barbed-wire fence in a combat situation, but still. [:)])
But the primary reason to choose the Phantom is that it's stronger, and that is important when it must be permanently attached to the barrel. Plus, except for some sparky Wolf ammo, the Phantom completely eliminates flash on my 14.5" barrel, so I have no complaints.
-Troy