League of Legends (LoL) Question: Master Yi and Hydra controverse

Master Yi doesn't really need the extra pushing power that Ravenous Hydra provides, on top of that he scales much better with Blade of the Ruined King, which is what I build on him 100% of my games on him.

Blade of the Ruined King helps his do more true damage from Wuju Style, increases his dueling power, lets him catch people out (since he has no CC built into his kit) and it helps him proc his passive too. It's just an amazing item on him.

It depends on what you want to achieve. If you're going for dueling and splitpushing Blade of the Ruined King probably is better. If you hang with team Ravenous Hydra is better in due to it's AoE damage.
Personally I go for hydra even if I'm splitpushing, because Master Yi jungle builds are destined to play with Enchantment: Devourer and also many people plays with Mark of Attack Speed combined with Greater Quintessence of Attack Speed - with your maxed ult it makes you have enough amazing attack speed. With such amount of attack speed I feel Blade of the Ruined King isn't that effective, as raw attack damage from Ravenous Hydra is more rewarding.