Nikon or Canon you can read reviews here
http://www.dpreview.com/
Once you have set a budget you really do need to get your hands on your shortlist. Image quality is good from either but ergonomics will play a large part in your satisfaction with the final choice.
When it comes to macro lenses I agree with postie that a prime rather than zoom will give the best image quality. Focal length will depend on your subject and how close to the lens it will be.