The relational model is much easier to implement (HN and most forums use it), and insertions are faster, but building an entire tree from any point but the root requires recursion. Nested sets are more difficult to implement and inserts and updates are more complex due to tree balancing, but getting the actual trees is simple at any level.
There is also the nested set model described here[0,1].
[0]http://mikehillyer.com/articles/managing-hierarchical-data-i...
[1]https://coderwall.com/p/ohomlg/nested-set-model-the-best-app...
The relational model is much easier to implement (HN and most forums use it), and insertions are faster, but building an entire tree from any point but the root requires recursion. Nested sets are more difficult to implement and inserts and updates are more complex due to tree balancing, but getting the actual trees is simple at any level.