I would rather have the consistent slow than higher DPS and a "chance" to slow.
Remember, even as a Bruiser, your goal is to prevent the enemy carrys from being able to get away or get to your squishies. Whether you accomplish that by CC or gap closers doesn't really matter, but you need to be able to do it, so you're pretty reliant on that on-hit slow. I wouldn't make it unreliable by going Tri-force.
I wouldn't want to go in to peel, and hit the guy but not have the slow proc. That could be a game changer since the bruiser diving your carry likely has mallet as well. So if they hit your carry once, they're slowed and can't get away.