If I download Unity 3.1 is it free, or is it a trial version?
Unity 3.1 is just an upgrade to Unity 3.0. The same licensing applies to whatever license you had for Unity 3.0
Both...it's free to use as you like, but if it's your first time running Unity, then you get a 30-day trial of the Pro features, which are normally only available if you buy a Pro license.