I have an Edge 510 and an Edge Touring. The 510 has very good battery life, and that was the main selling point for me. These days external power sources are cheaper/smaller/more convenient, so I might think about the Edge 20 if I was new to the GPS market. I don't use most of the 510's features, honestly.
I got the Edge Touring mainly for the maps. I used it this summer during my ride across the country. You can upload courses to the 510, but they don't work all that great and they aren't displayed on a map. The Edge Touring doesn't get great reviews, but I actually like mine. I planned on selling it after my ride, but I liked it enough to keep it for future trips.