Thanks for your replies. Here's more info about the race:
Its an outdoor race and the range is a couple hundred yards. I is also a winter race, so I need to be able to store the electronics in a waterproof container (like a pvc pipe) and still be able to transmit. I chose the 60mW because the range would be more than enough (even in a waterproof containter) and I think you want some extra range as opposed to edge of the xbee's range.
I was thinking I would use an uno, shield and xbee at the start to send out a "start timer" message when a wire was tripped. (Maybe using a door alarm or something). The second uno/shield/xbee would send out a "stop timer" message. Then, the explorer/xbee would be a USB interface to connect to a pc and receive the messages.
What will the Arduino's be doing? You may not need any Arduinos (or shields) at all. The XBees have some digital IO pins, like the Arduino, and can read, and transmit, switch info directly.
The Arduinos are for controlling the message sending. I thought it would be easier to program the arduinos to send the messages because my background is in computer science and not electronics. However, if you think I could easily do it with pins, I'm open to that.