Luxurious means (1) marked by luxury or (2) characteristic of luxury. Luxuriant means (1) characterized by rich or profuse growth, (2) producing abundance, or (3) excessively florid or elaborate. So luxurious often has to do with monetary wealth, while luxuriant describes types of abundance that do not necessarily relate to monetary wealth.
Examples
Luxurious
A cashier from the West Midlands who stole £1.7m from her employers, funding a luxurious lifestyle, has been ordered to repay some of the money. [BBC News]
The tower’s most luxurious units have not yet been revealed but analysts believe rents could be set as high as $15,000 a month. [Daily Mail]
She initially moved with her brother and mother to New York and lived in the luxurious Waldorf Astoria hotel for several months. [Bucks Free Press]
Luxuriant
It is clean and comfortable but lacks the aesthetic charm Bali is famous for, such as luxuriant tropical gardens and rooms with homey rustic touches. [Sydney Morning Herald]
The males sport luxuriant flaxen hair which almost hides their peanut-shaped black faces. [Independent Online]
One fine day, mom P and calf Peewee were cruising south in shallow water off luxuriant foliage covering the nearby shoreline. [Tampa Bay Newspapers]