I do not have Thunderspine and I personally love the Blightfang but based on the stats, higher critical strike rank and thrust boost, Thunderspine should do more damage. Besides that it is really a matter of preference between broadsword and cutlass.
When you want to use the weapon skill "Cursed Fire" for the Blightfang Edge it will take longer for you to make the attack causing you to take extra damage in combat, rather than with the Thunderspine Sword which will mix in a lightning strike with your hack attack. Plus the Thunderspine has a greater value in gold and is legendary. So it would be your best choice. Feel free to post more of these for me to choose if you need to.