It depends. Zpak is a brand of Azithromycin which is a cousin of erythromycin, both members of macrolide class. Levaquin (levofloxacin) is a brand of Levofloxacin which is a quinolone & cousin of Ciprofloxacin (cipro). Both are good against some but have different effectiveness w/other bacteria due to different mechanisms of action. For upper respiratory infections like pneumonia, they should both be fine. But c&s is ideal.
Depends on infection. Different antibiotics kill different kinds of bacteria - there are many 1000s of different species of bacteria. Different antibiotics also have different abilities to get into different parts of the body, such as inner ear, brain and spinal fluid, or urinary tract. This is why it is always very important to consult with a doctor about what antibiotic to take, on what schedule, and for how long.