XBee Pro Module - ZB Series 2 - 63mW with Wire Antenna.

Series 2 modules cannot talk to Series 1 modules so if you already have some S1 type XBees you may want to stick with them. The S2 modules are not necessarily 'better' than S1 for many projects. They're just different as they use the "ZigBee" wireless stack instead of the 802.15.4. This makes them better for low power usage and advanced users who want a mesh topology (many XBees in a spread-out configuration) - but they are more difficult to use for basic point-to-point setups.
This is the PRO Series 2 ZigBee protocol 63mW with wire antenna. Its good for point-to-point, multipoint and mesh networks. This module is a little more difficult to get going than the Series 1 - you must set up a "coordinator" module so they are not as plug-and-play. Adafruit suggest this module for those who are following the Building Wireless Sensor Network book as it focuses on Series 2.

Main Features

• Indoor/Urban: up to 300 ft (90 m)
• Outdoor line-of-sight: up to 2 miles (3200 m)
• Receiver Sensitivity: -102 dBm