Despite a whole lot of bitching and whining and fighting between Canon and Nikon fanboys, they tend to stay pretty close together in lens performance. If Canon releases a new version of their 70-200 which is much improved, you can be sure Nikon will move quickly to catch up. They're very competitive.
If you're a general purpose shooter, you're really not going to notice the difference between Canon's 50/1.4 and Nikon's, for example. There are of course exceptions. And if you have very specialized needs there are things to consider. For example:
Despite the high quality and popularity of Nikon's 14-24/2.8, it has taken Canon a very long time to release a competitor in the 11-24/4 [which you could argue still isn't a direct competitor due to the aperture]aperture and understandably exotic price]. If you shoot indoor sports Nikon's 200/2 is better than Canon's. If you shoot architecture Nikon has no competitor to Canon's 17mm tilt/shift. Nikon has no (modern) f/1.2 lenses because the F mount is too narrow. Nikon has not updated their 135mm in a long time and it isn't competitive in resolution or AF speed with Canon's (both are slaughtered by Zeiss's new 135 APO, if you don't need AF).