If you want navigation, creating a route on a computer and downloading to the device is by far the best. I don't know of a device that would enable you to create a reasonable cycling route right on the device. That's not what they're for. Any of the Garmin Edge models 705, 800, 810, on up. would be fine. In general, each iteration has improved the breed. They give turn-by-turn directions from a route that's been loaded onto them. Battery life is not an issue, because they'll all charge under way off a USB stick. They aren't great at getting you back on route if you go off, mostly displaying an off-route message. However one can get back to the route pretty easily by using the installed map which shows your location, local roads, and your route. Touch screen map is nice.
RidewithGPS.com is the best routing software I've found. I'm I'm home, I'll often print cue sheets from RWGPS, sometimes nice to have. I used to also have a separate wired bike computer, but Garmins have become so reliable that I've given that up.
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