The number of megapixels only really comes into things when talking resolution and light gathering.
Its always a trade off between the two.
The Nikon sensor has greater resolution, thats without doubt and in good shooting conditions that can be a real plus point, the downside is that those pixels are a fair bit smaller than those on the canon sensor.
Larger pixels gather more light, they have a larger surface area so get hit by more photon's.
Think of a pixel as a bucket, the bigger the bucket, the more light it can gather, hence better low light performance and less noise in those conditions.
Given the kind of shooting conditions we face here I would rather have fewer large pixels than one with over half as many again crammed on the same size sensor.
For me its not so much about the make of camera as it is the sensor itself.