If your house was built before 1970, then you probably have the capicity to handle the extra room. I would use galvanized metal pipe with fiberglass sleeve insulation to run the air to the room. It is smooth on the interior and allows the air to slip right on through, whereas the flex duct has a spiral wire through it and serves to slow the air down and wear it out.
I have seen a 30' run of flex duct that would not move cigar smoke if held 1' in front of the wall register. I had the flex changed out for pipe and the same run would blow a candle out at 1' away from the register.