Regarding controlling remote power sockets: you are correct that the functionality hasn’t been added to the OSPi software yet. Also, OSPi doesn’t have designated pin headers to solder an RF transmitter, so you will need to solder some wires separately to connect a transmitter. If this functionality is important, you should probably consider the Arduino-based OpenSprinkler.
In general, the Arduino-based OpenSprinkler is targeted to typical customers who want an out-of-box working solution, and OSPi is targeted to advanced customers who have experience with RPi, want to tinker with it, and possibly modify the software to extend its functionality. Arduino-based OpenSprinkler also has LCD and buttons.
Regarding relay: on both versions there is a built-in relay, and the software supports the relay. So you don’t even need to hard wire an external relay. In the next revision, however, we are going to remove the relay from all versions, because it doesn’t seem to be that useful, and the original plan to use it for garage door opener would require additional components (like a reed switch or IR switch to read back the status of the garage door). So I feel the relay is a feature rarely used, and should be removed (although the software support can still remain).