[PHI 3000] One's modus ponens is another's modus tollens

Some (e.g., David Chalmers) argue roughly as follows:

1. If physicalism is true, then zombies are impossible.
2. Zombies are possible.
3. Therefore, physicalism is not true.

In reply, others (e.g., Daniel Dennett) argue roughly as follows:

1. If zombies are impossible, then physicalism is true.
2. Zombies are impossible.
3. Therefore, physicalism is true.

Both arguments are valid. Which do you find more convincing?