I've only ever done NaNoWriMo and it's great because you know you're doing it with plenty other writers. Through Twitter and other social networks, it allows you to interact with them and a lot of published authors also take part it in to give encouragement and advice. Agents and other industry people also take part.
It's also a great way to start setting goals/deadlines/etc since that's how most writers are when working: they give themselves schedules and NaNoWriMo is a great starting point for that.
Hope this encourages you to try it this November!