Wifly or Xbees for wireless Arduino
Xbee shields easily act as cable extenders, you can have them both setup to communicate with eachother and just send simple RS-232 data over them. With a WiFly you are instead going to need to have the wifly act as a server or client in an exchange over TCP/IP.

I would suggest the XBee for their simplicity, but WiFly will allow you to go to a completely different location and use the internet as part of your range extender. If you have no way for the WiFly to connect to a router I think the options are limited here, you really cant use the WiFly at all. They are very different devices and this is almost apples and oranges.