1) One of the provided design might be bad and so people will choose the other one. For example text balloons background is slightly grey and the space between the comments is bigger than the separation line on the minimal chrome. Also the separation line is the same color as the code example and the attached image border, which kinda makes a different perception of the whole content. So here I would choose balloons, because of this flaw.
2) The dummy content might look better in one of the two designs, which doesn't mean that the concept of the case is bad, but just the rendering. ( This can be solved by providing a design + content for the two cases that makes them look equally ( another subjectivism ) good. )
3) Misunderstandings about the question. What is being asked here : Which one of those two designs is better? or Which concept is better?
If the former, I'd say balloons, as there's very little other grouping indicators present in the individual comments (and the page design looks pretty flat in general).
If it's the later, we'd need to consider the design of the elements of the page and the relationship between various headers and text elements before a good decision could be reached. As well, more color/size/contrast elements in a potential design may force the balloon elements to be of a much higher contrast themselves, which could ultimately overload the page.