Upload & Sell: Off
First thing you need to do, is start shooting. Pick a system and go with it. Right now, you don't even know your own needs, how can we help? One brand of camera isn't better than another, its just 'different'.
On that note, I personally use Nikon. I like the camera selection and lens selection for my needs, but you won't ever hear me parading around saying its better than Canon/Sony/ect. It's just my personal preference to use that brand.
This is also very true and same with what TrenchMonkey said. I originally read "macro" and automatically thought of the Canon MPE, which I do not believe there is a Nikon equivalent, but upon further thought, "reptiles and amphibians", there isn't as much a need for high magnification (5x lifesize of the MPE). I do know that Canon has a 180mm 3.4 or something macro lens, not sure about the Nikon side. Either way, people love the Sigma 150mm and Tamron 180mm which are available to both systems anyways.
I believe OP'll have to be more specific as to which Nikon and Canon body rather than just the brands. If it was between the 1DIV vs a D3200, the answer is obvious. If it was between a 5D3 vs a D800.... well we're not gonna start that up here/again. I do believe both have their uses and are more similar than different anyways. I believe an old adage was "get the camera that feels most comfortable in the hand [and menu system comes intuitive to you]."
I do believe budget is a really important factor though, not to mention "acceptable" IQ. I personally think it's awesome when people mount a 600mm Nikon onto a J1 or V1. I think the image quality is just dandy, but I know true wildlife shooters probably disagree.