How do you choose which ancient civilizations to teach and what key points to focus on for each?

It's time for Era 3 in my classroom, and I have about 14 class days (including an assessment day) to teach this Era (I am using the older Era pacing this year). I always run into the issue of whether to spend about a day on a major ancient civilization or have a PBL where the students explore ONE ancient civilization in huge detail. I have experimented with both and still feel the whole picture is left out no matter where I go.