I have owned both the Canon 70-300 IS and the Nikon 70-300 VR. They are comparable in range and both have very good image quality for the price, although in my opinion the Nikon is a decidedly better lens.
From an IQ perspective, the Canon is a bit more consistent out to 300mm, while the Nikon is outstanding up to about 230-250mm and then drops off a bit. However, the Nikon below 250mm delivers a sharpness and punch that was never obtainable on my Canon. Furthermore, and more importantly, the front element of the Canon rotates during focus, while the Nikon focuses internally. This is a big deal if you use circular polarizers, as the rotating front on the Canon makes CPs very difficult to work with. The build quality of the Nikon is also superior -- the materials used feel better and the zoom ring is significantly stiffer. The Nikon also comes with a hood and pouch, while the Canon does not. Lastly, the Nikon has a newer and a bit more advanced VR system. The Nikon offers up to four stops of hand-holding (equivalent to the Canon 70-200 F/4L IS,) whereas the 70-300 IS offers up to three stops.
If you have a few extra hundred dollars, I would sell the Nikon 70-300 VR for as much as I can get and purchase a Canon 70-200 F/4L IS instead, which is better optically than both 70-300s (and is of significantly better build quality than the Canon 70-300 IS.) I purchased the Canon 70-300 IS before the 70-200 F/4L IS existed, and if I were to go back to Canon today I would not buy the 70-300 again. If Nikon had a 70-200 F/4 VR that was as good as Canon's, I probably wouldn't have the 70-300 VR either -- but I am considerably happier with the 70-300 VR than I was with the 70-300 IS.