I used a solid core door 2" thick, mounted on an exterior frame--so it seals pretty well. I otherwise have very good soundproofing--decoupled walls and ceiling, three layers of drywall w GG, etc. The door is clearly the weak link, but not terrible.
How deep is your entrance space? I used a double wall construction for the room, so I am considering adding another door (probably one with a glass window) on the "outer wall" portion of the entrance opening--allowing me to have a more attractive view into the room when I want the entrance to the room closed, but don't need the significant isolation that the solid door offers. I just find the room too isolated when my kids are in there and I am not--especially since I have boys who will soon be inviting over teenage girls
Although not offering significant benefit on its own, using a second door of some type should temper (even if ever so slightly) the airborn sound leaking through the solid core door when they are both closed. There will basically be a 6" air cushion between them, although some flanking will obviously occur.
If you really want to go nuts, do two facing solid core doors.