Purebred cats are the most expensive, and are more likely to suffer from various genetic ills. However, there's no research to indicate that their life expectancy is any shorter than that of mixed breeds. There's no average life expectancy for cats of specific breeds. Housecats that never go outdoors will live longer than outdoor cats, and a cat lost outdoors will typically live only two to four years. Most pet cats are mixed breeds, commonly dubbed American Shorthair or American Longhair.
Mixed breeds make excellent pets, no better or worse than purebreds.
It's just a matter of taste.