I've used both the 12-24 and the 11-16... for video neither of them are particularly better than the other one IQ wise... stopped down to F5.6 or below, and they are both just about as good at equal focal lengths. distortion control on both is pretty damn good.
The biggest thing is that the 12-24 is slower, but honestly... for followcams, you're definitely not shooting wide open ever (unless you're weird), and for landscapes/walk around, the 12-24 just has a lot more use. You can use the 12-24 as a walk-around lens without an issue, but the 11-16 is UWA only, and you're paying for the speed to go along with it.
If you already had the 17-50 then sure, the 11-16 would be fine, but coming from where you are, the 12-24 would be a way more useful lens - not just for followcams, but for everyday use as well. One of my best buddies brings just his 50D, a 12-24, and a 50 f1.4 when he travels. Pretty much all you'd ever need, honestly.
Have a nice day, Bluename.