On the quality front, in general, it will be better to do it in camera when the sensor is particularly subject to hot pixels (those that only appear stuck when heated) or when there is other heat related noise introduced in the image. At that point, the current conditions in the camera are more accurately represented by a dark frame taken at the same time. There are limits there, of course, in that a dark frame taken for an extremely long exposure (e.g. an hour) might have more noise from heat than the prior image and so result in incorrect noise subtraction.
Now, newer sensors are improved in this area versus the older ones and so if the camera isn't showing signs of being particularly sensitive to heat related noise, then the advantage of taking dark frames at the end of your activities is more photos. So, there's clearly a time advantage there and it will be successful in dealing with an fixed-pattern noise associated with your specific sensor.
Edit from the comments... The out-of-camera option can be improved significantly by use of bias frames and dark frames. Bias frames are images with no exposure times and so only capture noise from the surrounding electronics. Dark frames, as you know, are long exposures with no light. Averaging bias frames to create a master bias and averaging darks to create a master dark reduce random information from individual frames and improves the quality of the DFS activity on the final image. Without that, you will actually introduce noise to the final image.
As an interesting side anecdote, Pentax used to force DFS for its cameras when they were still using the Samsung sensor as it was notoriously noisy in these conditions. They only made it an optional feature when they made the switch to Sony sensors. So, depending, on your camera body, you may not actually have a choice.