If you are willing to run wires, you might be able to do something like this relay board with the moisture sensor. I don’t think any programming would be required, just hook it up like the diagram shows and set your schedule. When it’s time to water, the relay board will get energized and if the soil moisture is low enough the relay board will open the sprinkler valve. Pretty slick.
To be honest, cost is the biggest thing I’m struggling with now. It’s far more expensive than I want and the single highest cost item is the moisture sensor I have chosen. I do think it’s the right sensor though. From what I’ve read about the sensors of the type you have linked, they’re not durable. I’ve never used one though.
I want wireless because of the flexibility and because wire is expensive too. I’m going to be using it in my gardens and I like to move stuff around. I might want it in one garden one year and another garden the next or in the tomatoes one week and the dahlias the next.
The XBee has four analog to digital converters – which basically means that I can read 4 values at each remote station. The moisture sensor will be one of them. Battery voltage will be another. I was thinking about soil temperature and a light sensor for the other two values but I’m kind of focused on the soil moisture for now.