While Nikon introduced the first serious digital camera with the D1, Canon quickly caught up, and for most of the early part of the last decade Canon sensors were widely considered to be better than Nikon. This caused a large number of photographers to migrate to Canon and a large percentage of shooters at sports events would be sporting white lenses. Nikon managed to turn this around with the introduction of the D3, and since then, Canon has failed to keep up with Nikon camera sensor quality. Nikon uses mostly Sony sensors, but also other third parties as well as their own sensors. Canon makes all their own sensors.
This trend is clearly demonstrated by the DxO camera sensor marks. While DxO marks are a little controversial because they do not factor in overall sensor pixel counts, they are the best comparative tool we have available and I find the DxO results closely mirror my own experiences.
The trend lines show that 2008 was a pivotal year.
Here are all the gory details in tabular form. I have included the consumer DSLRs as well in the table.
While Canon is clearly losing the sensor quality battle they are also increasing prices, with the 1Dx and 5D3 more expensive than their Nikon alternatives. I hope Canon turns this around as we all benefit from a competitive market place.