Hi Pieter,
The reason to not choose for option 2 are performance related risks. Always use option 1 if you don't need to navigate both ways.
To read more about associations, check this article: https://world.mendix.com/display/refguide4/Associations
And this forum topic: https://forum.mendix.com/questions/1821/What-is-the-navigability-property-of-a-reference-set-for