I am trying to make a new kind of forum and I am confused how should I go for displaying topics. Is it advisable to go for the former? What are possible UX concerns with it?
Each of the two has it advantages and disadvantages.
This layout issue is similar to text layout, where a single column text of is more readable - you don't need to move your head and eyes as much (down and to the beginning of the line), since there are less rows, however, the multiple text columns layout looks more aesthetic - it appears like less text. Relevant articles  
Relevant diagram in case the links break or in case you just can't be bothered to open them :-) :
From a responsive point of view, it is best to adapt the content (number and width of columns) to the actual window width and zoom, that way it looks good whether you have a full screen window, two windows side by side, a tablet in landscape mode, a tablet in portrait mode, a phone in either and so on.