60CSx here
As far as I can make out the 60CSX/Vista HCx and the 60Cx/Legend HCx are pretty much equivalent pairs, and the differences are mostly down to size/weight/cost/layout. The 60CSx has some extra bells & whistles, but they aren't really relevant on a bike.
The 60CSx is bigger than the Etrex series (though not by an awful lot if you exclude the aerial), heavier, has a slightly shorter battery life, has a bigger screen, and has the buttons on the front where access is easier.
As has been said, the 605/705 are more training tools with added navigation, rather than an outright navigation tool. Also the battery life would probably limit you to 200k on the audax front.
They are also fairly new, and I get the impression that not all the bugs have been ironed out yet, so you'd want to keep an eye on the firmware downloads.
I was in the pub at lunchtime a few weeks ago showing a non-GPS person how the 60CSx worked. I was on familiar territory and had only been using it as a moving map, so I set up a route on it to navigate home, by way of a demo. A 705 owner who'd been earwigging piped up and said his 705 didn't allow the setting up of routes away from the PC. I don't know whether that's true or if he just didn't know how, but if it is it's a pretty big black mark IMO.