the simple answer is BRAINS. What you require doesn't require having them. So arduino, plus has better power consumption.
When thinking is needed, then you use the pi. I use arduino to collect sensor input, and output, and pi to process it. This allows less chance of catastrophic failure if a power relay shorts or the a/c system motor spikes. the local arduino will take the surge and protect the main pi controller. Plus I only need to send a command signal that fire was detected and the local arduino will handle the cascading lights on stairs, and send elevator up so I don't need to worry about the house power going out, and the elevator will still work to go down in. The pi can decide when rainfall isn't enough and turn on sprinklers. It doesn't need to control them, it delegates authority. Long term I am looking at thousands of sensors for everything, you don't want to bog down the pi monitoring for snipers in the trees that may only happen once every 8 years when a crook is in office. But it IS needed to calculate trajectories for home defense system to take them out when they are detected. Arduino while nice can never cope with surface to air defenses to take out helicopter gun ships. Star wars is way over it's pay grade. but quad core 900mhz can do the math and track them. These new iphones themselves look like a good platform for defense automation as well. that way you can cover your retreat. Alas the apple store hasn't been able to answer my questions about gyro sensitivity for use as a gun sight controller, and i'm not blowing $700 to find out it won't do the job when there are $50 sensors I know will work.