OK, so I have both....
[Sorry for the long post - but working in a camera shop, I too get bitten by the "buying" bug. But I hope that my thoughts will help, and give pause as others decide "which way" to go.}
I started with having a Canon Rebel Ti when I was ready to jump to digital. So I got a 10D, and then followed that up with the Rebel XT. All because I had some lenses already. That grew into a system with a 12-24, 17-40, 28-135IS, 28-75 Tamron (for speed), the 28/2.8 and 50/1.8 for low light) and the 75-300IS. Along with two 420EX's and the ST-E2 for wireless flash.
Come late last year/early this year, I was thinking of getting a "super zoom" camera like the Panasonic FZ series for my planned trips this year - to have a light weight one lens solution. That was until our Nikon rep came in with the Nikkor 18-200VR just before Christmas. Man, such a sweet lens. Sharp, good close focusing ability, and a super VR/IS capability. In the store I was able to shoot at 5.6 @ 1/30 fairly consistently!
Had the chance to buy the 18-200VR BEFORE the popularity made it go on back order. And had the $150 rebates available for buying the PictureMate and the D50 together. So I jumped at it. Add to that Nikon has the 10.5 fisheye (that can be corrected for distortion in Nikon Capture), so I splurged for that lens too - always liked the semi-fisheye effect.
So I now shoot both Canon and Nikon. Its been only three months in having the Nikon D50 in my gear box. And I love it. JPEG images are truly ready to print. The 18-200VR is a truly great travel lens.
I will probably be paring down my Canon kit. Already swapped out the 12-24 Tokina for Canon to Nikon. Most likely will sell off the 10D and the Ti, they don't see much use - though both are great cameras. As well as thinking of the 28, 50, and 75-300IS. Why? Cameras are tools. You get the tools that will best do the job that you need it to do.
But for me the Nikon D50 and the 18-200VR replaces my need for the 75-300IS to a degree. And in some ways because of the 4 stop ability of VR, the 28 and 50 Canon lenses. Though I have sights on a great Nikkor 35/2.0 for really low light.
But based on my experiences recently, I would be very tempted as new purchaser to go with any Nikon DSLR with the 18-200VR for "general" shooting. I say "general", for while the 18-200VR is a sharp lens, it is not like the 17-55/2.8 or the 70-200VR/2.8; or the Canon equals.
Add your choice of an ultra-wide zoom (my choice is the Tokina 12-24), a 35 or 50 f/2.0, and for adventure the a semi fish-eye (hope that Canon offers one soon for the 1.6x factor). As a person that loves wide angle, these are my choices. Otherwise if you are more telephoto, a 200 to 400~500 zoom lens would be of more use than the semi fish-eye.