When using Composer to install something, how do I know what version to use?

When installing stuff with Composer it usually asks what version you wanna use.

You can see the different versions available if you locate the package on https://packagist.org.

For instance, the widgets package.
https://packagist.org/packages/silverstripe/widgets

It has 2 versions:

1. dev-master (almost always available)
2. 0.1.x-dev (in this case a SS 3.0 compatible version)