The fuji PXZ4 controller is a little 48mmX48mm controller that pusles a changeover relay.
There is an advantage in using one solenoid in that it doesn't have to be interlocked with a second valve. Two valves also have the tendancy to "fight" each other.
The disadvantage of any on/off controller that is being used on a quick responce process is the relay works hard "or chatters". This isn't a problem if the flow through the valve is throttled to ensure that the system is less responsive. In addition it is wise to use a solenoid with a low current rating(1Amp).
Fuji coutrolers are available from WWW.coulton.com