Having used many different field guides, I would like to give my impressions. I have chosen these books because I own many of them and can vouch for their quality.
The National Audubon books contain mostly color photographs, whereas the Peterson guides are mostly line drawings. I use them in conjunction with each other, especially for the identification of smaller plants. Having a color plate is nice to have visually, but the line drawings show us important details and differences from one plant to another to help us distinguish between closely (and sometimes poisonous) related species.