It will always come down to a number of factors...
1. Who is going to be spotting for you and are they an idiot?
2. Are we talking hypertrophy or strength?
3. Are you prone to injury?
... and so on...
Every person is going to have different set of answers here. However I personally would lean towards the bar.
Lets be honest here for a moment, since some idiot made the first set of pink dumbells with 0.5 on the side, I'm sorry, I feel betrayed, there was no need for it, no-one doing 0.5 on anything deserves to be in a gym.
Dumbells switched sides, they cheated on the iron soldiers with the flouncy abercrombie wearing, mobile-in-the-fricking-gym generation. They gave birth to the squat rack curlers, the old ladies who stand around infront of the mirrors, the guys with the missing lats, the idiot with the perma belt who tells everyone how to leg-press, 'don't squat, its bad for your knees', everything we despise was caused by those dumbell touting idiots wanting to open the sport up to a bunch of tards for a profit.
So don't forgive them so easy, they started this.
Like that ex who cheated on you once, you might accidently go back every now and again, after all he/she serves a purpose, but by God I am not letting that cheating bitch take the place of the most faithful thing in my sport - have you ever seen a pink barbell with 0.5 on the side???
- so yeah use the little bitch when it suits you, sometimes she might put on a nice 70kg jacket, but damn we know where she has been and we know where are heart lies.