User blog comment:Supp0rt4n00bz/Welcome to the League of Hydra!/@comment-1330314-20131219173311/@comment-1330314-20131219185745

The extra 25 AD and 6% lifesteal on BT have a combined gold value of 1164g. Against single targets, Ravenous Hydra's active is the equivalent of an extra autoattack every 10 seconds (assuming you land the ability at point-blank range). Spread over its cooldown and factoring in the AD it gives, this is about the equivalent of 7.5 bonus AD. You'd need 248.3 attack damage for the active to begin outscaling a fully charged BT in gold value. This is assuming you spec into Offense and not Utility (which would reduce the item's cooldown), which increases the AD BT gives by a larger amount than it does for Hydra. This is also ignoring any potential AD scalings the wielder might have, which would factor in more from a BT than from a Hydra.

Though effectively Ravenous Hydra can give better burst by itself, BT is strictly better in combat on average, particularly as Hydra's AoE only affects tightly clumped groups of enemies. The advantage to Hydra is that, in addition to the good stats it gives in combat, it allows for better waveclear and farming. If you want to pick an item specifically for combat, you're better off building a Bloodthirster.