Two devices are a network, just a minimal one 
My $0.02 can be read here, FWIW.
If you go with S2 (now called XBee ZB), the low-power wire antenna model is a good one to start with, part number XB24-Z7WIT-004. XB24-Z7PIT-004, XB24-Z7SIT-004, XB24-Z7UIT-004 are equivalent, they just have different antenna options. The Pro models feature increased range and require significantly more power.