When does one use "는데" or "지만"?
Besides the ones mentioned, are there any other words to express "but"?
Which one of these is used more commonly in conversations?
How does one decide when to use them?

Question

Answer

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.