The 360 slim is basically the elite with $200 worth of extra features for the same price (N wireless adapter, and 250GB included, all for the same price of $300). The PS3 slim is basically a smaller, slightly more constricted version of the PS3. If you are going in comparison to their predecessors, then the 360 slim is better. Other than that, their tech/price are now basically equal (except for bluray which can hold more data, at the expense of brutal load times) so your choice will depend on the exclusives you will enjoy more.
Also, if you don't own either, then you may want to take future tech into consideration (Xbox Kinect (high tech ripoff of playstation's eye toy), or Playstation Move (high tech ripoff of the Wiimote))
Own the PS3 slim, and the origional elite w/250GB HDD and N adapter.