The light year by far. It is equal to about 63,241 AU's and each AU is about 93,000,000 miles (the distance from the earth to the sun). Over all a light year is close to 6 trillion miles.
The Astronomical Unit (AU) is the average distance between the earth and sun ... about 93 million miles. In terms of the speed of light, that's about 8.3 light-minutes. One light-year is equal to...
Generally, they aren't - the two terms are too clumsy to try and use together. The "Astronomical Unit" is the distance from the sun to the earth, or about 8 light minutes. You can see the clumsiness...