Don't get me wrong the PS2 is a great system, but it's now moving to the past. The 360 is where the majority of game support is.
As for trading in a 360 for a PS2 or even a PS3, the question you have to ask yourself is... are you willing to part ways with all the games that are coming out for the 360 in the summer and fall? Does the PS2 or PS3 have any games you'd be interested in at the end of the year. What about exclusives that are important to you.
Im lucky enough to own all the Current Gen and Next Gen systems. I am not biased to one company. I perfered the PS2 in the last Gen Era, but as it's starting off I perfer the 360. It's all about Game Support. Sony used to have it, but now Microsoft is getting 2 fold over the PS3.