A) If your choice is between doors and the curtains, then solid tightly closed doors will be a better option. This is especially true if you are dealing with two rooms such as a  living room and family room (where appearance and convenience also counts).

But — the curtains will do the job too. You can install the curtains tightly, with no cracks between the curtains and the walls. The curtains come in 48 inch width.  If your doorway is 63 inches, you would need two curtains (48 x 85 inches each) so they can overlap each over and can be installed above the door frame.