Now for pure distance, the P900 might be okay, but I have looked at alot of moon images from that camera when I was trying to decide on the SX60HS. Nothing stood out as really great. However to get that reach with an APS-C, you are going to have to run a Sigma or Tamron 600mm along with a 1.4x, which increases the costs substantially. So for a moon shot or shots of jetliners, the Nikon would win until you put long glass and a quality TC on the SL2.
This is with the 7D2, but the SL2 would produce the same basic image.
Is the Nikon better here? Tough to say without shooting in the same conditions to compare apples to apples. But you don't have to buy $900 in more glass to get the shot either on the SL2.
Now if you are willing to go with the additional expense, and look at the crop out of a moon image, the SL2 will likely keep details, where the Nikon will start to fall apart.