Just shuttling data hither and yon does not take a lot of capacity, unless you need to significantly process it as well.
Another reason to go with the Mega, though, might be that you have more than one serial port. Using one for an Xbee, and another for the Kangaroo, would allow you to put the microcontroller on the 'bot instead of on the laptop side, which means you get faster response to things like IR sensors or whatnot.
(I'm assuming the laptop is staionary, whereas the bot is moving, when you're saying "receive commands from the laptop.")