Canon would be better for zoom, Nikon for macro. I use Canons myself. You can get a good Canon 20D on Ebay that will let you do most anything you want and it will be fairly inexpensive for a prosumer SLR. Unless you are going to make posters or crop the heck out of your pictures, the megapixels will be fine. And you'll have a great number of lenses to choose from depending on what you want to do. Good luck to you, let me know if I can help.
www.fredmiranda.com has some very good reviews.