2D is meant for noise reduction of static objects and 3D is meant for noise reduction of moving objects.
But like everything it comes down to your field of view and other parameter settings.
In theory having both allows for stationary objects to run a different noise reduction and allow 3D to just focus on moving objects.
Some can drop 2D to 15 and 3D at 30 and get great captures while others have to have the numbers be the same, and everywhere in between.
Like everything, play with both and test it with moving objects. You should take NR down as far as you feel comfortable