This requires a much lengthier response than this one. But here is the elevator pitch.
In the 1980's, Nikon had a commanding lead over Canon. Most Pros used it and as such, there was a ton more information on how to get the best out of the system. Fast forward to Digital, and Nikno found itself out-gunned by Canon who due to its other divisions, had far more assets to through into the game including CMOS sensors they had designed. They also got an amazing leg up with image stabilization which they got into many of their lenses. Nikon countered with VR technology but it was not widespread until recently. In addition, Nikon lenses tended to be more expensive than Canon in the super telephoto range.
Today, the race is much closer. Nikon finally managed to get good sensors and bodies to support them. If I were to net it out (note that I am NOT a Nikon user), here is where it is at:
Nikon:
+ Better noise performance although both systems are incredibly good compared to point and shoot consumer cameras.
+ Better wide angle lens performance. Canon just doesn't make a good wide angle lens. They have redesigned a couple of them recently but still not quite there.
+ Better flash support. Nikon just knows better how to expose flash images.
Canon:
+ Wider set of lenses with image stabilization
+ Lower cost of lenses in some cases
+ Dominant in Pro market although the screw up with Mark III autofocus system (multiple recalls) set them back fair bit. Still, Canon now has the position Nikon had in 1980s.
+ Multiple bodies with full size sensor such as 5D Mark II that you list.
+ Throws in software that converts raw images. Nikon charges for it. That said, the Canon software is pretty bad from usability but still, if you don't want to buy other software and want to shoot in RAW mode, Canon is there.
+ Canon simply has more R&D horsepower than Nikon so I expect them to keep producing more.
+ If you go with Canon, I can help you with answering things. If you go with Nikon, you are out of luck
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I think your son's advice is right on. I have 1Ds MKIII and it is a wonderful camera. But it is very large and heavy and you have to be in a serious mood to shoot with it. I also have a 5D (MK I) and I take that around most of the time and it produces wonderful images. BTW, used 5D are pretty cheap and great alternative to 5D MKII. I have posters at 16x20 inches produces from it!
If you go with Canon, I highly recommend getting the 24-105 IS lens. The combo will take care of 90% of your pictures. Then add 70-200 F4 IS and you now have that range covered. Each one of these is another $1K or thereabouts. If you let me know what your interest/budget is I can suggest other choices.