They are both excellent systems but as a 2nd system to buy the Xbox 360 has two key advatages.
1. You have friends with Xbox 360s, if you go online theres no point in being lonely so that alone is worth its price of admission ($40 per year)
2. A wide range of cheap games from Live Arcade to a huge back catalogue. Games are the biggest cost of a system so simply that fact will reduce your costs substantially. Also you can borrow games from friends if they let you.
A few side points, because it uses batteries you won't have to worry about turning your system on and finding the controller discharged and needing it to be plugged in and since you've got batteries for your Wii its an easy fix to whip them out of the Wiimote and into the 360 controller.
It will look nice against the Wii, and Wii60 has a very nice ring to it, don't you agree?
Tease.
