> I've always wondered, which is better: a 7.1 system or stereo headphones with "3d audio"?Stereo headphones all the way. You only have two ears; they can only ascertain direction based on a) the amplitude and phase differences between the sound reaching each ear and b) the frequency and time-domain filtering effects of your outer ear. These effects can be very accurately emulated using a physical head model or digital signal processing. Properly recorded and mixed binaural audio will provide the most convincing spatial effect, because your eardrums can't tell the difference between a binaural recording and actually being in an acoustic environment.
>And if 3d audio is so great, why doesn't everyone just use stereo speakers (maybe with a subwoofer)?
Your living room isn't an anechoic chamber, so you're hearing both the sound of your speakers and the reverberation of your room. You can tell that you're listening to two sound sources positioned in front of you, because that room reverberation provides substantial spatial information about your actual acoustic environment and degrades the spatial information of the recording. You can mount speakers really close to your ears, so you only hear the sound of each speaker with one ear without the colouring effect of your listening environment; we call such an arrangement "headphones".