It seems like your dilemma is about lenses. About versatility of the Nikon 18-200 vs. image quality of Canon "L" lenses. I had the 40D with the 24-105mm "L" lens, and now the Nikon with the 18-200mm. Maybe when looking a charts there is a difference in image quality. But with the 18-200mm, I haven't come across a real life situation where I thought "gee... look at that 18-200m distortion and lack of sharpness" or whatever else that might compromise image quality, and where I thought that I would have been better off with the Canon lens. And I'm sure the opposite would hold true as well (if I had now the Canon and earlier on the Nikon).
Honestly, what I remember from the Canon lens is how beautifully-made and smooth it was to operate, and the fact that the (24-105) had image-stabilisation, which I really liked. As far as image quality goes, I looked at different test pictures taken with both the Canon and the Nikon, and I couldn't tell the difference between the Canon and Nikon lenses. No charts here, no pixel-peeping, just looking at the picture to see whether it was sharp and gave me that 3-D, better-than-life image that I like. Both the Canon and the Nikon gave me that.
The big thing about the 18-200mm is its versatility and vibration-reduction feature, whilst producing image quality that I like. Full stop. The only drawback (again, in real-life, not looking at charts) this lens has in my opinion is barrel-creep when you hold the lens pointed down ( and I guess pointed up) while it is zoomed. The barrel will move due to its own weight. Not a biggie for me, because in the rare cases when I had it like that while taking a picture, I just held it. But I think that Nikon could have done better in this case. The build quality is, of course, not on par with the Canon "L" lenses or the Nikon pro lenses, for that matter, but for the price you cannot expect it to be. It's not cheap, but it's not in the "upper" category of prices either. The Nikon 18-200 is not as well built as the Canon "L" lenses, but then if you take a gold-ringed Nikon lens, I'm sure it'd feel just as nice.
Also, if Nikon only had one lens to sell - the 18-200mm - and Canon had only to sell the two lenses you mentioned, then maybe I would think twice which to get. That's why I like the 18-200mm. I will probably buy more lenses in the future, but I will always keep the 18-200mm because it's a keeper, a very good all-rounder. It's a good foundation upon which to build. I'm not 100% sure at the moment which way interests me the most, landscape, macro, astro, photo journalism-style?...Until then, the 18-200mm suits me perfectly. Then, I will get the mother-of-all specialist lens. What I'm trying to say here, is that whatever lens you buy, it's not like it's the end of the road. Image quality in real life situations will not be an issue, whatever lens you get. So get the lens(es) that suit you now.