DBs will be much easier to hold onto and walk with, even your heaviest DB.
the farmers walk event come in so many different configurations, some have thicker handles, some hang low, others hang closer to your hands. But all will have the weight spread over a much larger surface area than DBs, and therein lies the key difference. That weight will want to wobble and move around on you, so there is a tremendous strain to stabilize the weight while at the same time trying not to loose your grip and move the Farmers down the course.
try using a reg Barbell for Farmers, you'll notice the difference