FWIW I have owned both systems for an extended period of time and have had many opportunities to compare them.
When considering selling a PS3 to fund an Xbox 360 be aware that the Xbox 360 is NOT a quality system. What I mean by that is although the games are great, the controller is great, and the xbox live service is wonderful, the overall build quality of the system is quite frankly garbage.
I have owned two xbox 360's and both had RROD within 2 years of service. The systems were well maintained and taken care of FWIW.
Now supposedly the issues that the old models had were supposedly fixed with the new slim, however, there have been some reports that some of the main components thought to be responsible for some of the RROD errors on the original 360 are still being used in the new slim. I can't confirm whether that has any validity or not. However, my brother happens to own a new Xbox 360 slim, and although we have not encountered any hardware errors, the overall quality of the system is no where near as good as the PS3's, and the price difference is negligible between the two, some 360's depending on the HDD space, is actually more expensive that the PS3.
It is this shoddy build quality that keeps me from buying another Xbox360 myself, because experience showed me it's not a quality system, which is unfortunate because the system really has some outstanding qualities.