RG6 long term is better. The highest quality 300 ohm cable has less loss but this is more in theory. Non careful instalation of twinlead (allowing it to be close to metal or the ground are bad) also when twinlead is wet the loss goes up considerable, and twinlead doesn't last as long- it eventually dries up and cracks- especially bad when wet. 1 thing to be careful about especially at UHF frequencies is the choice of matching transformer. Most home antennas are 300 ohm so you'll need a matching transformer. Be sure to get a weatherproof model and one that has minimal loss- 2 db is a good one at UHF.