The soundstage is badly distorted on headphones; Pure left sounds only to arrive at the left ear, and are perceived as being way to the left, with nothing on the right to create a semblance of a credible space. Likewise on the right side.
Pure left sounds on speakers also reach the right ear and they are perceived as being on the left side of a stage, far more realistic.
Sound from speakers at an identical left and right level, say, a vocalist or lead instrument, iff* you are seated exactly the same distance from both speakers, will deliver a nearly convincing center image - except that, again, there will be sound coming from the left speaker reaching the right ear and vice-versa, so that center image will be poorly focused at best, and only to a person sitting directly in the middle. Anywhere else, the center image fails. Try this: play a tune with a vocalist singing in the center - sit right in the middle. Now hold a record jacket, edge to you nose, blocking the left signal to your right ear and vice-versa. You will immediately hear the singer pop into focus, more accurately and realistically, coming from the center.
With headphones, a sound at equal levels left and right will be solidly placed in the center. Also, headphones will be immune from room resonances and reverb.
So - which is better?
For critical listening, for just one person, I would say a good pair of headphones - with one important condition: Unless the musical source is binaural - and that can sound spectacularly realistic, you need to temper that exaggerated soundstage.
It’s not too hard to do.
You need a separation or blend control to narrow that space. On some recordings, like some early stereo, where there was an exaggerated separation for effect, that will make the music far more enjoyable. But I find it essential with just about any stereo source.
It could be as simple as putting a 5 to 50 Ohm variable resistor (depending on your cans’ impedance) between the left and right wires the headphones.
I have adapted a few cables with this simple mod.
Then you can dial in exactly what you perceive as a natural sound stage, and the center will always be far better focused than any pair of speakers can ever hope to achieve.
You will be amazed how much better headphones will sound.
* iff - that’s not a typo. Look it up.