I think the differences are too many for a simple answer.
The P900 is likely a nice camera. The big differences that pop out are sensor size, no changing of lenses and the P900 has an electronic viewfinder.
I think the only way to do it would be get one in your hands somewhere that would let you step out the door and shoot and maybe allow you to use your own SD card so you can see the pics on your computer.
If you like the feel of the D50, I would think one of the newer DX bodies would be the way to go.