Which is better Nikon or Canon? Why?

For SLR cameras

It really just depends what your looking to do with it. I have a NIKON, a Fuji, a and Canon and i love them all the same.
Canon is amazing for low light shots, while NIKONs are wondeful high speed action shots. Lenses also effect your camera performance too.
Is say decide what kind of photography you want to do, then decide what camera model would suit that best, then find a few lenses to help you achieve the best shot.

Both, neither

Which is best for YOU is something you have to determine by research on each companies website and visiting a camera shop and holding each in your hands.

Entry level? (under $1, 000) Nikon D5100 or Canon 600D/T3i
Advanced dSLR? (over $1, 000 with advanced features) Nikon D7000 or Canon 60D
Professional full frame? Nikon D3s or Canon 1Ds, Mark III

Which you choose will have to be based upon your needs as a photographer and the available lenses from each company.

In the same price ranges, they are pretty much equals. One may have a feature the other doesn't have. Quality wise, they're on par with each-other.
I'm a long time Nikon owner, but I can't say anything negative about Canons.

Both are excellent

Nikon is better than Canon.
And Canon is better than Nikon.
And trust me it's true.