Champion dependent. Also depends on which camp you're talking about. Lots of things to consider. Max on-hit damage vs. monsters from BotRK is capped at 85 (25 + 60 from passive) but the attack speed increase gives it more DoT compared to Hydra. Hydra is return has AoE passive and the AoE active. End result is that - with all else being equal - Hydra is much better at clearing the regular camps, BotRK is a bit better than Hydra at Dragon and Baron.
But all things aren't equal. Champions with good, reliable attack speed steroids but not much in the way of AD ratios (Xin Zhao) will clear better with BotRK. Champions with good AD ratios but bad/no attack speed steroids (Kha'Zix) will clear better with Hydra. It's pretty irrelevant in the end though. Clear speed as a determining factor of which items to buy is pretty low on the list.
With all that said, on Sion I suspect SoteL -> Brut -> tank -> finish brut into either Ghostblade or BC is the way to go unless you're quite fed early on where Hydra or Trinity Force (paired with a heavy CDR build) become optional. Sion's attack speed steroid is unreliable since it's attached to his death passive and the max. 180% AD ratio on Q is deceptive. It's not particularly that good for something that requires a 2s charge.
Well yeah, I was talking specifically about his Sion built. ;o
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