If you can afford Hydra I personally like hydra. It allows you to clear faster and it let you cancel your ridiculous long W animation. If you notice when you W you can't move... I mean they can't move for like .75 seconds, but you can't for .50 seconds.... (1.5 seconds for Fury W, and 1 second for you).
Hydra eliminates this problem just W (hydra active) and boom YOU'RE FREE!
So yea I prefer Hydra over Black Cleaver. But sometimes you know your team needs a tank asap (so you don't have time to get the 3K + gold for Hydra). So in these situations I like to go Brut for that cheap price then go defensive. I rarely finish off the BC until late game. Basically in those situations I go Brut > Defensive items X3 and if it gets to late game I will finish off the BC. Then maybe throw in a LW if the game gets to the point where I have 6 items. So if I go the BC route it usually ends up with boots, 3 defensive items, BC, LW (aka you have armor pen so your ADC can wreck havoc).
I don't have specific champion where I go "Must go BC, or must go Hydra." It just feel out "do i need to clear these minions and roam, or do I want some armor pen and CDR to all in RIGHT NOW."
For example a good case where I normally go brut is like maybe I all in at level 6 and get a kill. Go back to base with like 1500+ gold. I know if we all in again I'll most likely win. My team hasn't lost towers so I know lane phase probably going to go on for a few more minutes. Why not grab a brut and try to get another kill?
Hydra example would be "oh crap Mid lane is 0-2." I'm winning my lane but I'm not strong enough to diver under the tower w/o jungler. Let me grab this tiamat push this wave and let see if I can help mid, or ping my jungler over to dive top (I know I can't dive alone, but I also know I can dive 2v1 since I'm stronger than whoever I'm laning against).