Learn to monitor and control your home & environment with self contained, inter-communicating microprocessors. Applications include ham radio, robotics, weather stations, model railroading, toys and more. - KK4HFJ
Well that question is like asking what's better, a hammer, or a saw? It all depends on what you are trying to accomplish. If you are trying to read analog and digital inputs, make a decision, and control a device, the Arduino is the clear winner. If you need to run a linux operating system, with web and database services, full screen displays, and keyboard and mouse input, then the Raspberry Pi is more appropriate. There are many projects that are best served with a combination of the two. For instance, we use a network of Arduino's as sensors, feeding a database running on a Pi, and other Arduino's are picking up jobs from the database to be executed. This could not be done by an Arduino or a Raspberry Pi alone. So don't limit yourself to making choices, take both home, thay are small!