I'm not sure how true this now, but a few years ago the idea was that in general Canon had the edge (and it does have the marketshare) but Nikon was better at low-light, non sports, and had somewhat better quality for the price, whereas Canon was better as sports, video, and had a larger range of lenses. This is why I chose a Nikon D7000 about 7 years ago. If I knew what I knew about my preferences now, back then, I would have gone with something lighter and smaller, like a Pentax or Olympus.
If I were to choose today, I'd go with a Sony mirrorless instead since that seems to be where the market is going and Canon/Nikon are slow to catch up to mirrorless cameras. The nicer Sony A series mirrorless cameras are perfect for my use cases (I don't do sports, and generally like low light or underexposing my photos, I'd like to have full frame, and I really prefer something light), whereas fitting these criteria is difficult/not possible with Canon or Nikon, primarily the competing lightness and full frame aspects.
If I were to choose today, I'd go with a Sony mirrorless instead since that seems to be where the market is going and Canon/Nikon are slow to catch up to mirrorless cameras. The nicer Sony A series mirrorless cameras are perfect for my use cases (I don't do sports, and generally like low light or underexposing my photos, I'd like to have full frame, and I really prefer something light), whereas fitting these criteria is difficult/not possible with Canon or Nikon, primarily the competing lightness and full frame aspects.