For 3D mixing,you need to have a bit of both I think.For example,when I'm working on panning and stereo width,I always prefer my headphones,however when I'm pushing along the depth dimension,I always prefer my speakers.But then again,you can work anyway you want,but I think at the end,when you think it's finished,you should always have a listen on headphones,your studio monitors,sometimes even your laptop speakers or the speakers of your phone.You just want sort of standard-ise your mix for all platforms.