I am building an Arduino system to control servos on a model railway. I have built a few "slave" boards with an Atmega 328 on stripboard (veroboard) and I am using a Mega as the "brains" of the system. The control panel switches will go to the Mega and it will send a serial message to all of the slaves telling them what to do.
The slave boards are identical and will have identical programs - the only difference will be the board ID which can be set with some jumpers.
I chose the Mega because it has lots of I/O pins and because it has multiple HardwareSerial ports. It will also be controlling the speed of the trains and taking wireless data from the hand-controllers for the trains.
For a smaller system you could use an Uno or Leonardo or Micro or Pro-Micro in place of the Mega. The Leonardo has the advantage of a Leonardo or Micros is that they have a spare HardwareSerial port.
...R