I know they're different in effect.
However, Jinx's passive pulls off what Varus' should do.
When Jinx gets a kill, her passive gives her a massive boost that either makes her untouchable or capable of running past a turret to catch the next kill. She gets this huge burst of MS that grants her control and threat in a fight.
When Varus gets a kill, his passive makes him attack slightly faster, giving him slightly faster Blight procs and DPS. If this burst of AS was noticeable, I'd be fine with that, but it's barely more effective than Ardent Censer.
With the way bonus AS works, a 20% AS steroid on Varus for 3 seconds basically means he gets in 0.4 more basic attacks over that 3 seconds. +40% over 6 seconds is about 1.58 basic attacks over that 6 seconds.
Meanwhile, Jinx's passive grants +175% MS that decays over 4 seconds. Since we don't know the decay rate, if we just assume it decays evenly, it's effectively as strong as a 4-second, +87.5% MS steroid.
That's right, while Varus' passive is almost worthless compared to an item, Jinx's passive makes her Hecarim on-kill.
Again, the effects of the individual passives are different, but the effectiveness is what makes Jinx's passive fantastic, and Varus' passive garbage.