Technically, BoTRK with its passive will out damage Blood-Thrister in terms of raw AA damage if the enemy has more than 1000 health. Anything beyond 1000, BoTRK starts to outshine BT significantly.
Of course, that is why I personally run BoTRK the majority of the times. The active is great for a clutch save, and its build path does not require large purchases. However, BT is better than BoTRK on champions such as Riven who doesn't AA as often, but still have high AD scaling abilities.
I also consider a BT if I am really ahead, have already gotten a BotRK, and if I am a ranged champion. The enemy team will likely focus me in team fights, and the extra life steal and shield can really come in handy.
If I am melee, I would just pick up an hydra. Hydra is probably the BEST lifesteal item in game. It gives great wave clear, a great active, and almost as much AD as a BF item.
In my opinion, BotRK is far more superior than BT. However, both are still great items.