I didn’t buy it yet.
I’m waiting for an answer for my second question which is the most important.
I’d go for the 24VAC version.
If i don’t have any clear answer i’ll go for plan B : Zwave relay switches like in this tutorial :
http://domotique34.com/votre-arrosage-automatique-domotise/
Around the same price as Opensprinkler for my installation and i can control everything through Domoticz.
Just more troublesome to install (hardware not software).