As a quick summary: the OSPi kit provides the 5V power circuitry, shift register and solenoid drivers, RTC, ADC, relay, enclosure and terminal blocks. For software development, an RPi is all you need. But the extra hardware is necessary to control actual sprinkler valves.
For comparisons among the different OS versions, please check this summary by Doug:
viewtopic.php?f=8&t=703