For top end pro lenses I'd say there's no real difference but with cheaper consumer lenses I've found Nikon lenses generally a little better. That's just an impression from using lots of both in real world shooting.
in most cases you get what you pay for regardless of make. you can get crap lenses or excellent (and usually expensive) lenses from both manufactures. one of the ones i know that's relatively cheap and pretty good is canon's 50mm lens.
All averaged out there's little difference. Both manufacturers have some great and some poor lenses. It's no longer the case that you need to buy first party lenses to get the best results either - Sigma and Tamron both have some absolutely fantastic offerings that are also cheaper than the Canikon's equivalents.