I've seen lots of questions about this lately so here's how I see it.
Canon is the number one camera manufacturer today. There is no doubt about it. It has done so using a worldwide multi-million dollar advertising campaign. They do make great dSLR's and their point-and-shoot cameras are some of the easiest to use.
Nikon is older than the leading brand. They also make great dSLR's that have a technical feel which attracts mostly professionals. Their point-and-shoot cameras aren't as good as their dSLR's however and tend to be cumbersome to use and easily fail.
As to lenses, they are more or less equal in quality and in choices. With respect to compatibility, both have their limitations especially when trying to mount an older lens model to a newer body. It is worth noting that Nikon has at least made it very clear that their entry-level models will only autofocus a couple of lens model lines.
Overall, when it comes to dSLR's and lenses they are the same. It is in the point-and-shoot cameras where Canon cameras prevail.
The world is not divided among these two brands. There is Sony and Pentax for those who yearn for more value for their money. To those who are dying to know which is really better, look up Hasselblad. It's the industry standard.
Now you know!