I can't answer your questions specifically, but I do own an eTrex Legend and have used it mounted on a mountain bike. The eTrex series doesn't have audible signals, so I'm not sure how it would alert you to the turns. The Vista has a barometer in it and it allows you to get a compass reading while standing still.
The eTrex series is pretty old tech by GPS standards. I think you could get something much more current that would do a better job, and probably not cost a whole lot more.
Also, I had problems with my Legend. After awhile, the constant shock of hitting bumps made it cut put periodically, rendering it basically useless as a bike GPS. Not much cushion on the eTrex mount. Bottom line, they're basically hiking units.