Bloodthirster or Deathsdance on Fiora?

is a better generalist item. It is better if you aren't csing well or are behind. The reason being that it's build path is a lot easier as well as offering some CDR along the way. The most expensive purchase along that build path is pickaxe which costs 875. The next most expensive is the combine cost which is only 625. Being able to build multiple longswords into an item is a huge help if you are starving for gold.

is a little more situational but CAN be better if gold income isn't an issue. In this build path, you have to make two major buys. B.F. Sword (1300) and the combine cost (1,150) and this item offers no utility in the middle of the build path.

is arguably a better duelist item is arguably a better laning/split pushing item. It will help you stick to lanes significantly more as long as there is something to lifesteal off of. It is also helpful for dueling if you are able to take advantage of the over heal but grievous wounds can make the life steal negligible obviously.

I say all of this assuming you already have Ravenous Hydra and a 20% CDR item like Black Cleaver or Trinity Force.