Depends on what you want to do with it.
Basic cycle computer functions, plus GPS, but hardly any mapping, a refurbished 305 for $135 on ebay is nice.
If you want to use it for navigation, you really want the 705 (or the new 800 with Touch screen)
If you want to train with power, you need the 500, 705, or 800.
So: 305 equal basic
500 power (i.e compatible with an ANT+ power meter), but limited navigation
705, power, and navigation
800, power, navigation, ad touch screen.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.