As a dual wielding fury Warrior with Unbridled Wrath, I can say that I'm fairly certain that it does proc of "yellow" damage as well.
About your question, I'm no big expert about Warriors, but if you go 2 handed fury, I'd say Improved Demoralizing Shout is your safest bet IF you use it every fight. That's just my guess, I will use some numbers to further push it, but I might be missing something more fundemental, or plainly be wrong.
Unbridled Wrath should decrease the attack power of your foe by additinol 120, which translates into 120/14~8.57 DPS(Without any skills that is), if you have about 40% damage reduction from armor and go on berserker stance you'll get a total 5.65 DPS reduction in the fight, with dodge/parry/miss it's probably around 5 DPS. So every 15 seconds of a fight, this talents buy you 75 damage, if you took ~600 damage that fight(pulled that number off nowhere), you decreased it by 1/8, that's quite nice, should get you an extra couple of fights before you rest. This is for PvE targets that hardly increase their base DPS by special attacks, against PvP targets it's different, some melee PvPers have higher "yellow" damage than "white" damage, so the decrease should be higher. And to top this off, bear Druids and other Warriors rage generation will further decrease with the improved debuff, while that shouldn't be really a problem for them(they will gain alot from damage you'll cause obviously), it might help a bit.
Unbridled Wrath, assuming you hit your foe 10 times(actually hit, crit counts as one, missed/dodged/parried doesn't) should give you 4 extra rage per fight(if the number really is 40% as I remember it to be), if you execute that's irrelevant as Execute will eat your extra rage, if not, you should get around 1 extra mortal strike every 8 fights, not game breaking. I think Booming Voice might even save you more rage :) . Again, in PvP the sheer amount of damage you'll get will make you have all the rage you need, so I don't think this little extra would make a huge difference. Might be missing something though.
Then again Unbridled Wrath works against all classes, Improved Demoralizing Shout only works against melee fighters(And Hunters).