Roman, the number of pixels isn't important for planetary imaging. The size of the pixels may be. Your setup will project an image of a certain size on the sensor chip. If the pixels are smaller this image will cover more pixels and so there is potential to capture more detail.
If you want to capture larger areas of say the moon the bigger chip will be better, but otherwise for planetary where it's virtually impossible to cover the smaller chip you are better of with the 21. Speed in FPS is very important too in planetary imaging as it allows you to capture more data for better noise reduction etc.
If it's planetary you want to do get the 21.
