I'd stick with the VistaC over either of those. It may not have been specifically designed for cycling, but it's much preferable if you ever run into a storm and need to find a shortcut home, or if you just get off the proper route and need to figure out how to get back to it.
The 305s are unique in being GPS devices that can't provide the basic navigational function of telling you where you are. Plus the battery is non-replaceable and I wouldn't count on it for an extended days ride.
I agree with Peter. These units aren't designed for navigation. In case it helps though, I've consolidated a fair amount of info on both these units on my blog...