I prefer Nikon's as they use ED glass at their low-end. You have to go with Canon L at the higher end to get nicer lenses (though Canon's got some good lenses at their low end, put I prefer Nikon's "low end").
I think where Canon might be a touch ahead in the lens arena is with the f/4 L zooms (17-40 f/4L, 24-105 f/4L IS, 70-200 f/4L (IS and non)), and the ultra fast primes (24 f/1.4L, 85 f/1.2L), which Nikon haven't really had good equivalents for. I think Nikon get this, hence the new 16-35 f/4 VR and 24 f/1.4