I was also looking at both the Polar CS200 & Garmin 305. I bought the 305 and have had no regrets. It is a fantastic unit - & one I'm yet to fully utilise.
Advantages (for me) of the 305 over the CS200 were
1. The 305 connects to the computer via a usb cable (absolutely no problems with uploads), while the CS200 uses sound (little 'beeps') which requires you computer to have a microphone (& there are reports also of some people having problems with this type of upload).
2. The 305 stores & uploads all your ride real time data while the CS200 (I think) only uploads the post ride averages. This is especially useful when comparing your improvements overtime & putting together training programs.
3. Software available (e.g. SportTracks & Garmin's own software) takes advantage of the GPS component, automatically usinging google maps - once again great for post ride comparisons etc.
4. Wireless unit on the 305 is apparently less prone to interference than the CS200 - can't vouch for this, but I can say that I have had no problems with either the HR monitor or cadence unit's wireless reception.
5. Although bigger than the CS200, the 305 is still surprisingly small in size (I was expecting it to be bigger).
6. The 305 has some great training extras e.g. Virtual Partner (which I'm yet to use but looks like it will come in use) which the CS200 does not have a similar function.
Anyhow (if you can't tell) I'm very happy with the 305
