Kevin~
As others have said, either decoy will probably work. All of my Brant shooting experience is on Long Island's South Shore bays. For many years, I gunned with 10 Black Duck decoys and 10 Canada decoys. Brant came consistently to that rig.
On the other hand, I have hunted Brant that were very trusting and those that were very wary and difficult to stool. Adults are much smarter than young-of-the-year birds - so hunting Brant is tougher in years with little production. As with anything else, the most important thing is to be where they want to be when they want to be there. For hunting Brant over decoys (i.e., not pass shooting), that generally means being on a shoal or bar that will put the bottom in reach of feeding birds once the tide drops. Flagging and calling (just use your Canada call) can turn indecisive flocks, too.
And, as much as I enjoy hunting Brant - for their calling on the wing and the graceful motions of the flocks as they approach - I have found nothing as exciting as Broadbill. It's a nice dilemma to have!
All the best,
SJS