A coupe is of a closed-body style, 2-door car, often sporty in nature. A coupe generally has either 2 seats, or 4 seats placed in a 2+2 configuration, meaning that there are only 2 seats in the rear, smaller than average whereas Avantgarde is a sports version of a coupe.

Avantgarde features a more sporty appearance. For the Avantgarde trim level, both cloth seats and vinyl come standard with leather optional. The front of the car is a little different, as well; the grille features three bars with the three-pronged star logo set into the grille. It is designed and intended to appeal to the widest range of luxury coupe buyers.

The Avantgarde features larger alloy wheels, a lower sport suspension, and improved calipers for the brakes; the interior also features a three-spoke steering wheel.

Sources:

1. https://www.picknbuy24.com/column_178.html

2. http://www.automoblog.net/2009/02/12/coupe-vs-sedan-whats-the-difference/