It's primarily about the performance at my location. Sling had buffering issues during primetime. Hulu was worse, and their interface was unintuitive (I believe it's being redone, however). Youtube never had buffering issues. I do prefer Youtube's DVR feature, but that wasn't the dealmaker for me.
But that's just at my location, and I wouldn't extrapolate this to be the case everywhere. Anyone interested in streaming services should do the free trials and see how performance compares on their ISP.