I wouldn't use an adapter though. If feel is most important for you, go for the Canon, if image quality and exposure/white balance consistency is most important, go for the Nikon.
Why would you not use an adapter? Most, if not all Nikon lenses made after 1959 have aperture rings and the Canon lenses don't, so for a macro shooter, an adapter and reversed Nikon lenses make the process a lot easier and more controllable. Older, used Nikon lenses can be had for good bargains.
I don't know that I'd automatically hand IQ/exposure/WB to Nikon carte blanche. Nikons typically are better in low light shooting (less high ISO noise) so a win for them there, but not across the board. IQ is more about the lens and the shooter. Heck, I could even argue that post processing skills play a big role as well.
I have seen the posts stating Nikon's top-of-the-line glass is the best, period, but is it really? I dunno, I bet the Nikon shooters think so.
Then again, Canon's top glass looks every bit as good from what I've seen. So do some of the lenses from Sigma or Tokina.
It's a Ford/Chevy/Dodge argument.
We've seen posts in this very forum from users with top-end cameras that make you question if the photo was really shot with that camera and we've seen shots from users with older, low-end cameras that are just absolutely superb in all aspects.
Don't get me wrong, I was all set to go Nikon and I actually prefer the controls on the Nikon, but macro is what I really like and I really want the MP-E65 lens at some point, so that leaves me no choice but to go Canon. The ability to use that adapter and reversed lens technique will make it a lot easier to do because of the aperture ring on the Nikon lenses. I'll be able to buy cheaper, used Nikon lenses and start shooting macro now and still have the ability to get and use the MP-E65 later (when I can afford it). I don't regret, nor worry about IQ or WB or exposure issues by going with Canon. I picked what will provide me the capabilities I want because I believe the quality of the output from Canon or Nikon are on the same playing field.