Whats the difference between P3e & P3ec?

The difference is on terminology, as far as I know. The core design and principle are same

Please take a look here.

P3ec has some layouts and text reports that have been customised to look like the default layouts and reports in P3.
P3e/c also has some default settings that are different to P3e, but these are customisable anyway.
If you are making a choice on which one to go for I suggest you consider the industry you work in. If your work has a strong Engineering and Construction focus then go for P3e/c, otherwise go for P3e (or Teamplay if you are in IT)

Jim