Slack all the way. Team 8 has been using it since Winter of 2014-2015 and it has basically replaced email.
Benefits of slack:
- Centralized communication with all team members and mentors, easily separating robotics and personal stuff
- Channels to organize discussion and keep related topics in one place
- Tons of integrations and features (google calendar, google drive, dropbox, github, etc)
- Slack Standard is free for all non-profits and heavily discounted for organizations affiliated with educational institutions
10/10 would recommend. If you want to try it out, slack is free with a message limit of 10,000 saved messages in message history, then unlimited when you begin to pay.