Garden/Greenhouse monitoring

[davidf]

SPI is good for very short distances, like between devices on a PCB. CAN is better for longer distances because of noise immunity (error correction). http://www.ni.com/white-paper/2732/en

Atmel has CAN-bus built into some of their processors, so you could build a board and still utilize the Arduino language for programming, or you could add a SPI-CAN chip like on this board:
https://www.sparkfun.com/products/10039 CAN would allow you to string a bunch of devices together, because they can be daisey-chained on the same bus. So you could have a 4-wire cable that provides power and communications out to several devices.

Ethernet (hardwired or wifi) is the best option if you want a bunch of devices that can talk to anything directly without a base station, but it will add to the cost of 'slave' devices and also to the complexity of the board (you have to pay special attention to trace routing). You can buy a Power-over-Ethernet Arduino board now though: https://www.sparkfun.com/products/11230 Combine the POE board with an 8-port POE switch and you could have multiple sensors up to 100 meters from the switch with one cable. Ethernet is also the best option for high bandwidth or high-speed data requirements (which you won't need unless you want to drop a video camera on the same monitoring network).

Bottom line (in my experience and without doing much research), CAN is probably the cheapest option but would require upfront engineering to figure out the firmware and hardware required.