If you had neither system, the decision is much harder and now only comes down to glass and nothing else.
Glass, and nothing else? I have trouble seeing it that way.
To me, there are a lot of other things to consider besides glass:
Familiarity with user interface. This is important to me, because I do not read manuals to figure out how to do stuff. I simply refuse to force my brain to focus and concentrate on something. So if I can't just pick up the camera and figure out how to use it within a few minutes of just "playing around with it", then that camera is not for me. Nothing in the world is worth the anguish that comes from making my brain work hard to learn something. If I were considering switching to Nikon, I would first have to pick up a Nikon and play around with it for several minutes, to see if the menus were able to be readily and easily figured out. Yes, something that seems so trivial to most would actually be a huge major consideration for me. This is my M.O. in many other areas of life, and I would not change my M.O. when it comes to camera systems.
Customer service. This is a big factor, and I am no over-rating it because it is so important that it cannot be over-rated. I don't know anything about Nikon's customer service, so I would have to do a ton or research to find out exactly how they handle this part of their business. The results of that research would be a major factor in deciding which system to use.
Widespread usage. How many Nikon users are there out there compared to Canon users? The more mainstream and popular a system is, the easier it will be for me to borrow or swap lenses and bodies with friends. It will also be easier to find used gear such as lenses, grips, and other accessories that are compatible with my system. For example, I own a Toyota Corolla because it is so common, and parts and accessories, both used and third-party, are plentiful and available just about anywhere at great prices because the supply is so high. If I owned a less-popular car model, I would not enjoy this smorgasbord of readily available, inexpensive stuff that fits my car. Also, just about anybody can service my car, because there are so many of them that everyone is familiar with them. The same can be said of camera systems.
There are yet more considerations, other than lenses, that are quite important to me, but I am tired of thinking and typing at the moment, so I won't be listing or discussing these other considerations at this time. Maybe I'll come back to this post and add them in later.
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