I made and placed a flag on one of my planets, and it allows me to teleport to it from my ship. On another planet I placed a teleport booth, and it seems to do the same thing. In what way(s) is(are) a teleport booth better than than flag? What are the differences between the two?
Once you've bookmarked a teleporter you can teleport to it as well as use it to teleport to other bookmarked locations or party members the same way as you would from your ship.
Once a flag is bookmarked you can teleport to it from any teleporter, but once you are there you can not teleport out.
Edit: It's worth noting that there is a difference between casual and normal in that you can only beam to ship from near the surface on normal. So if you are building under ground you cannot easily get anywhere else on normal mode (making teleporters more valuable).