It sounds like the instructions for the free trial download may be a bit out of date as all three versions are available for download, 2012, 2013 and 2015.
It is really up to you which version of the .NET Framework you wish to target. Normally users would only target an older version if that specific version was the only one available on their customer machines. Otherwise I would always advise using the latest release supported by the version of Visual Studio you are using.
