Most of the time, we use -는/은데 in conversation. -지만 is used in both formal and informal writing, but it's rarely used in daily conversation, or so I think. -긴 하지만 is probably more common, but -긴 한데 is still more common in this case as well. A : He's your friend, right? B₁: 친구긴 한데 잘 몰라요. B₂: 친구긴 하지만 잘 몰라요.