I'm actually wondering about poison. Why is deadly poison "better" than poison? Sure, it inflicts more poison, but at twice the cost. The poison to quanta ratio is dropped from 2 to 1.5 when upgraded.
The ratio is worse, but this is damage that's very hard to avoid. I also thought Deadly was a bad upgrade when I started. My first deck was Speed Poison. But fact is, both Poison and Deadly are cheap and can thus both be launched early in the game to start the damage. Death then has more expensive cards to make use of the accumulated mid- and late-game quanta, such as Bone Wall and Dragons.
So while it's more expensive, it speeds up the kill so much it's easily worth it since it's still cheap.
