1. I will leave (future simple)
a. future facts: The sun will rise at 6 tomorrow a.m.
b. predictions or expectations: He'll be there on time; he's never late
c. strong intentions: I will lose weight no matter what!
d. instant decisions/reactions about the immediate future: The phone's ringing. I'll answer it.
2. I will be leaving (future continuous)
a. events/actions in progress at a specific time in the future: By 5 tonight, I'll be flying over Greenland
b. predicted or expected trends: In 2050, most people will be driving hybrid cars
Hope that helps