While a catch can and an air oil separator both perform the same basic function - they remove oil from the air being recirculated in an engine - there are some important differences between the two.
An air oil separator is a more sophisticated device than a catch can, as it uses centrifugal force to separate the oil from the air. This means that it is more effective at removing oil from the air, and thus will help to prolong the life of your engine.
A catch can, on the other hand, simply collects the oil in a container, which means that it will need to be emptied regularly. While a catch can is less expensive than an air oil separator, it is not as effective at keeping your engine clean.