Etrex Vista HCx or Legend HCx and City Navigator maps are best for touring.
(Vista has altimeter/compass, Legend doesn't - only of any relevance if you are interested in getting reasonably accurate gradients or amount of climbing).
The Edge 605/705 is smaller and neater, but is primarily aimed at training rather than navigation, and it's got a battery life of 8-10 hours, after which you need mains for recharging it (difficult when camping).
City Navigator maps are cheaper on SD card than on DVD, but you can't use them on the PC for planning or reviewing rides, and you can't copy them so there's no backup.