First of all.. I'd like to say, this topic has been brought up countless times before on Head-Fi.
I really dont think the two are comparable. They both have their tradeoffs. Speakers have a more natural soundstage, music that you can feel, but they also have drawbacks like dealing with room acoustics, things like that. A good pair of headphones, in my opinion, as far as accuracy and resolution goes, will rival a good part of speakers costing vast multiples of its own price. It's a good way to get audiophile like fidelity, without spending a lot of money, since headphones are cheaper to manufacture. It's also a good way to listen privately, and I feel with headphones, that some music is just more intimate because theres little air between you and whats generating the sound waves. Headphones also typically seem to reproduce bass frequencies with a lot more accuracy then a lot of loudspeakers as well, since the diaphragm is so tiny, theres emmense control over its motion. The same cant be said for larger loudspeaker drivers. There are speakers, however, that reach a greater field of fidelity unrivaled by any headphone on earth. They cost a LOT of money, though.

The imaging of headphones is also somewhat weird, since theres very little, if any at all, crosstalk. This usually places the sounds inside your crainium, which can be irritating. Some headphones though, like the AKG K401/501 have a soundstage thats portrayed outside your head, almost as if they have natural crossfeed. Because of the lack of crosstalking in headphones, however, they will always have a more opaque image then loudspeakers.
Also, I'd like to add.. It's very convenient while listening to headphones if you're planning on relaxing, using the computer, laying down, or even falling asleep listening, which I enjoy doing frequently. This is nearly impossible using a single set of loudspeakers, however, because you've always got to remain in the "sweet spot" to get the best sound.