My wife has a Forerunner 305 (she runs, I don't unless chased) and I have an Edge 305.
The Forerunner does everything the Edge 305 does, but lacks an altimeter, so elevation/grade data is less refined (uses 3D GPS only). Also, the screen is a bit smaller, doesn't seem like much, but for me it means fewer fields per screen.
IMHO the Edge is definitely better for cycling, but you cannot put it on your wrist and use it to train for your next marathon.