Most of the photos on my site,
www.artandstructure.com, were taken with a Nikon D40 (the least expensive "current" DSLR to my knowledge), with the most expensive lens being a Tokina 11-16mm lens ($600 and highly recommended by the way). The Nikkor 55-200mm VR is my other favorite lens...and only $200. Yes, a 70-200mm VR is on my list, and so is a D90 in a couple months, but both companies make quality gear. Anyone quibbling over the slight variances from one to another needs to learn how to take better photos because the quality that can be had with either system, even on a budget,
is professional and any photographer worth their craft can make photos of the highest quality with these tools.
My photos are owned in at least a half dozen states and are on display at a gallery in Ashland, Oregon, considered just about the best small arts town in the country.
As far as who has more lenses to choose from, who cares? I suspect Nikon has a greater range of lenses available to current cameras than Canon if we want to include all the old lenses going back several decades. If we want to include current lenses only, then Canon may have the edge, but PCs have at least 10 times as much software available for a given task than Macs, yet I have the best tools IMHO and most of them are Mac only.
As far as market share, again, who cares? PCs have ten times the market share of Macs. See how much I care above. Nikon and Canon have been neck and neck, essentially splitting the market evenly for the last 2+ years (when counting just DSLRs. I have no idea what the split is for all digital cameras but it is outside this conversation though Canon makes great point and shoots). Installed base beyond that does not matter. A good 2+ year old DSLR can still take great photos but if we are going to quibble about it, a 2+ year old DSLR generally isn't in the same league as a current DSLR. It is a different generation of technology. I would also suspect most serious professionals (and even amateurs) are upgrading their cameras in 3 years or less anyway, so recent market share is a largely reliable indicator of any useful "installed base" for this kind of conversation.
In short, to answer the original poster's question: the differences, if any, are slight and could be adjusted in post anyway. Don't worry about it. Go shoot and enjoy. Whatever focal length/f-stop you need is most likely available to you. The rest is up to you to make use of.
For myself, the 11-16mm Tokina is a great lens. While Nikon's 14-24mm is generally considered the best ultra-wide zoom for full frame work, I really like the Tokina and it can supposedly be used at 15-16mm on a full-frame camera and is available for both Canon and Nikon bodies. I also have the 50mm/1.8. Canon also makes one of these though I hear the Nikon has better bokeh with regard to the shape of highlights due to a design using more blades. I don't know; I haven't played with the Canon. Both companies make a comparably priced 70-200mm with image stabilization. I am also considering the new Nikkor 35mm 1.8. Those lenses cover the extent of my wants/needs.
All the best,
Jesse Widener
Art and Structure design studio