Radios: XBees are made by Digi International and there’s plenty of kinds to choose from. We recommend starting out with the 802.15.4 XBee with a chip antenna. This will adequately cover a small indoor space, or a larger outdoor one (up to 100 meters). If you need more range, there’s the 802.15.4 XBee Pro which can cover over a mile outdoors. However better range comes at a price. The Pro version is slightly larger, somewhat more expensive and will cut your battery life to a quarter of what it would be with the standard XBee.
Antennas: If you’re planning to integrate your circuit into clothing, it’s probably best to use a radio with a PCB antenna (as opposed to wire). The PCB antenna has almost the same performance as the wire antenna, but it’ll stand up better to wear and tear.
XBee models: There are a wide variety of radios in the XBee series. Maybe a bewildering variety. For wearables projects, we recommend the 802.15.4 XBee or for more advanced projects that need mesh routing, the proprietary XBee Digimesh is available. XBee ZB ZigBee Pro radios are better suited for stationary networks, because the routers and coordinators that make up the network need constant power and benefit from a stable physical layout. There’s also 900MHz for North America and 868 MHz models for Europe that give somewhat better performance in noisy environments.
Look for more information on configuring XBees in Building Wireless Sensor Networks and Make: Wearable Electronics.