I have an old Magellan...
I had it mounted on the stem for the last 4 years, to replace bike computer and it survived several crashes (in one of which I broke my left shoulder

). Seems pretty safe there.
With around $250 you can by a Garmin Vista C, very compact, light and small, nice color display. It also has real compass and altimeter.
The Edge 305 looks definitely cool, but more for road training than MTB. I like to have detailed map loaded in the GPS, so in the end it replaces bike computer and maps
Display isn't too small and if you have the right screen on , you can easily read bike computer functions (speed, time, altitude etc...)
I need to upgrade mine, so I'm waiting to see the Edge 305 before I decide, but I most likely go with Vista C.