If you go Canon or Nikon you are getting a terrific camera/lenses. It is all the personal preference about the fell and the "system software/menu system" that drove me from one to the other.
1. Feel of the camera body in your hands is important, but make sure that you work with the cameras menu system. I started out with Nikon and switched to Canon and have never looked back becuase for me the menu system was more logical - for my way of thinking.
2. If you buy used and you can get good deals if you take your time looking, make sure that you get an accurate shutter count because each camera body has a "life to the shutter" meaning that almost all cameras are good for a minimum of 100k shutter actuations and there are lots of programs out there to help you get the facts.
3. I purchased my first Nikon system off ebay and I checked the feedback (only took those with 100%) and then I communicated with the guy selling the setup and he was terrific - ended up throwing in a few freebies since I was brand new to the hobby.
4. I am not comfortable with purchasing camera gear of Craigslist, purchased tanks and the like...but never electronics.
Here is a great site (IMO) to do some research
http://www.dpreview.com
Hope this is helpful.
I have owned a Nikon D70 and then a Canon 50D, Canon 70D and a Canon 60D....all great cameras, but they are APS-C ----meaning they are crop sensor cameras.
I currently have a Canon 6D and several "L" series lenses. When you get ready to ask about MACRO lenses...send me a PM or post a new thread and I will chime in. Hope this is helpful.