I have 30 years experience in high end audio. I go to several shows per year including international. Speakers are a huge interest of mine so I seek out and listen to every raved speaker, and walk the halls checking out every potentially great speaker that I hear any promise from. I've picked my favorites to own. Yet my first foray into headphones (Stax) has shown me that all speakers I have heard are more flawed than the headphones. That is a strong statement, but it is not born of bias when you consider that I own hudreds of thousands of dollars worth of speakers and yet still make the preceeding statements.
So, it comes down to, for me, more the pleasure factor of listening to headphones versus that of listening to speakers. For some reason it was very easy for me to adapt and I seem to get as much pleasure from the headphones, but I hear more information in a more coherent manner. Many questions about the scoring, particular instrumental sounds, the lowest level details that I struggled with before have been answered with the headphones.
For sonic quality, low distortion, the ultimate detail, in my opinion the headphones are far more sound per dollar than speakers.
Nobody can argue about personal preference, so if you are more comfortable listening to speakers, if the sound of an energized room is more "real" to you, if you love sharing with friends, if the "feeling" of bass over your whole body is a huge advantage, so be it: you may get more pleasure out of a speaker. But on a pure distortion, ultimate detail, seamless coherency basis I don't think a speaker is made that can equal the best headphones - except for the factors that can only be given by a set of speakers, that of being surrounded by sound, feeling the sound especially the bass, and "looking" at the soundstage in front of you in the room.
A strange thing is that my headphones cause me to enjoy my speakers more than I did before. It seems that having a transducer that delivers such extreme detail and coherency makes me accept the limitations of speakers, while also appreciating their visceral aspects that headphones cannot delivery.
For me, an "either/or" decision or piece of advice would be very difficult. I think "both" is the best situation: a great set of headpones and a great pair of speakers.