i've never used my polar F6 and Garmin 305 at the same time, but i entered the totals from my ride today and the total calorie burn on my Garmin was much higher than on Buckeye Outdoors. then again, who's to say how accurate Buckeye Outdoors is, since they use a formula to figure out calories burned based on your distance ridden. my totals from today were:

30.35 miles
16.8 average
2128 calories burned (Garmin)
1624 calories burned (Buckeye Outdoors)

almost a 500 calorie difference is a big difference...