Pretty simple difference: practical vs performace. I am practical so I use the Defender on all my cars. It is not a performance-oriented tire like the Premier...
https://blog.tirerack.com/blog/motorsport-messages-from-marshall/michelin-premier-as-vs-michelin-defender:-which-is-best
The Michelin Premier and Michelin Defender are two completely different tires meant for two different jobs. The Michelin Premier with its higher speed rating and softer tread compound will have more immediate response during lane changes and sharp steering inputs. It will also maintain grip longer during hard cornering on dry pavement. In exchange for the higher levels of cornering performance, the Premier has a lower treadlife warranty than the long-lasting Michelin Defender.
In comparison, the Michelin Defender is all about practicality. This tire is designed to get its driver from point A to point B thousands of times over. What it lacks in dry road handling abilities it makes up for in ride comfort and long treadlife. With its 90,000-mile treadlife warranty, this tire is one of the longest-lasting tires currently on the market. This is not a tire that will set any track records, yet sets its sights on a casual consumer who doesn't consider his or herself an aggressive driver.