The 2nd one. The one that looks like a thumb drive.
It takes a bit to get used to. But once you just use it and then randomly switch it off while watching a movie or something, you'll be shocked you ever listened to headphones without it.
Its fucking audio vudu magic and should not be possible. If you could wear headphones that you couldn't feel, you would swear you weren't listening to headphones. The sound would feel like they were coming from speakers or people in the room with you. Its nutsos.
To be clear... this isn't about "surround sound". It is about making headphones sound like they are coming from physical sources / speakers in your room... not from things attached / in your head.