It's actually not a brilliant idea to calibrate the pod on the track (not a tartan track anyway); over the distance (1200 metres plus) the slight resilience of the surface is enough to cause a deviation, as is the curvature of the track.
I suppose in the absense of a properly surveyed course you might borrow someone's Garmin to measure out a stretch of road for yourself. The longer the better. It is easy to calibrate and recalibrate the RS800sd on the fly, so if you started your course from home you could recalibrate every time you set out for a run, according to the shoes you had on or the pace you intended to stick to.
That said: I have just been out for a 10 miler with several k along a main road with kilometre stones, and the RS800sd was accurate to within a metre per 2 kilometres; my pace varied from 5:15 per k to 4:27 per k, and the road is nicely undulating. That was using the default calibration but after selecting the shoe (I wore the speed sensor inside the new Addidad shoe today).
I think the Garmin 305 and Polar RS800sd cater for two different groups of runners. If you are really interested in geographical data then the Garmin is for you. But if you are really into HR data and analysis and you want reliable feedback on your pace as you run, go for the Polar.
If I was rich I would buy the RS800sd and the Garmin 205, I think.
If I didn't have much time for computers and software and analysis but just wanted basic HR data and a reliable post-run record of where I'd been and how fast, I'd go for the Garmin 305. It'd give be the additional advantage of the "virtual partner", too, great if you do a lot of running on your own.