Get a Forerunner 301 from e-bay (around $150). It seems to me it does much the same as the 305, but without cadence and is cheaper. Tie it to Sportstrack on your PC and you'll have so much info you won't know what to do
I believe one of the differences between the Garmin Edge 305 and Forerunner 301 is the altitude measurement. The Garmin Edge 305 uses a barometric pressure measurement to more accurately determine altitude (and therefore, grade). The Garmin Forerunner 301 (like the Garmin Forerunner 201) uses the GPS signal for altitude. I have both an Edge 305 and a Forerunner 201 and the Edge 305 is significantly better.