If you're going to Chengdu and have time to visit the panda breeding center, then don't bother with the Chongqing Zoo. But if this is your only chance to see pandas, consider spending part of a morning at the giant panda and red panda enclosures. To get to the red pandas, go to the left of the English introduction board and down the stairs. The outdoor enclosure has a half-dozen extremely active small red pandas. The zoo has also brought in new animals including antelope, giraffe, and elephant in recent years.
Tip: The best chance of seeing pandas is during feeding -- between 8:30 and 10:30am. If the pandas aren't out, poke your head in the office behind the English introduction board and ask the zoo warden if he'll feed them. Sometimes that's all it takes.