funny, I had the forerunner 201, and have the 720i. According to what I am reading, the garmin must have improved hugely as the 201 was NOTHING in comparison to the polar. I have only occasionally (Read: once per month) gotten bad readings with my polar. There is an easy error correction feature in the polar software. The software is the reason I would reccomend the polar. Grade and altitude readings as well as elevation gain are extremely accurate. Not so with the 201 (maybe the 305 is better?). The 201 logbook software was a joke. The dropouts for the 201 were all over the place with only moderate tree and/or building cover. I will hope that the 305 has addressed this issue. Do you like GPS functionality? go with the 305. For heartrate training, I cannot imagine polar being beat.
*edit* In addition, as a racer, I see 720i and 725 all over the place in the peloton (there are coded HR belts). I have yet to see a 305 in a race.