Is this the type of digital sensor your were referring too?
https://www.sparkfun.com/products/13322
Also i guess i have answered my own question regarding the expand ability of the system, it can go out to 64zones (valves)
so the last question i guess for the moment would be
For the multiple Digital Soil Meters i described about would i be better to go the RPi version or the OS version of Open Sprinkler?
Chris