2d vs 3d Depth of Field What is the difference between Depth of Field rendered at render time (3d) compared to Depth of Field created in post (2d). Both are depth of field, they look very similar if not the same. 2d depth of field has the advantage of being faster to compute and being and you can easily change what part is in focus, whereas if you use 3d depth of filed and you want to change it after then render you have to start over.
I am primarily focusing on cycles
I ran some tests with 100 samples each with this scene:


Defocus Node:


Time: 2:01 + roughly 7 seconds for compositing = 2:08
Cycles DoF:


Time: 2:02
Conclusions:
Based on the above results, rendered DoF seems more reliable and realistic, as well as a little bit faster in this case.
The main advantage of composited DoF is the ability to tweak the settings after the render.