As Matt Field suggests, the thinner, somewhat less durable (but more pliable) stock used by Aviator produces a less expensive product (they are also not cellophane-wrapped like Bikes).
Due to their flexibility, Aviators -- also known as Mohawk brand in some areas -- fan and spread easier in some people's hands. They do become worn quicker than Bicycle cards, warping and showing the effects of oil and dirt.
As your hands become more supple and strong, your card preference will change. Also, various methodologies and handlings lend themselves to difference card stock (false dealing, palms, flourishes, faros, side-steals/passes, memorized decks, etc.)