Hi,
For the physical/electrical interface I'd use RS485, with a twisted pair cable and proper termination it'll easily handle the distance you mention. You could go the ethernet route but it adds complexity and cost - and requires at least 4 wires.
For the protocol I'd use MODBUS (RTU for RS485 or TCP for ethernet), it's a well established protocol with good (and free) documentation.

I don't have any experience with wireless so I won't comment on that.

/Henrik.