...getting both is a must.
If you can afford only one, the answer is dependent on what your priority is:
1. Stunning Environment With Water Effects That Will Rock Your World
or
2. Zombie Behavior That Will Raise The Bar To New Heights, And Ruin Your Thoughts Of Previous Models
While there's a huge library that comes with the Metro Pack, you'll still be limited to the same zombie look and behavior, that you're accustomed to.
The mob zombies in the Apocalypse Pack are alone worth the price. These crazed ladies and gentlemen can be used to populate any environment past, present or future, with incredible results.
Ideally, you'll want to own both of these must-haves...