I like Polar better than Garmin, but if you want Power on your device you will need to go with Garmin, as it doesn’t look like Polar has any interest in going with ANT+ compatibility. However, I think getting power on your device is minor feature. I like having a wrist device and a separate bike computer so it doesn’t bother me to not have power on my wrist.
All the newer Polars have user replaceable batteries, so that isn’t a problem anymore
Before Polar went with GPS you were stuck with just a foot pod, and I think if I had to choose between the two a foot pod is much better than GPS, because you get much more accurate real time pacing. The newer foot pods are very small and light and you won’t notice they are even there. Admitably GPS is probably more accurate to measure the distance of a course especially a course with not too many sharp turns, but the new Polar RS800CX has a GPS pod available that can be used to measure this and at the same time use a footpod. This is great for keeping the pod calibrated. The Garmin Forerunner series can also work with a very small and light footpod.
Basically other than Power, the top of the line Polar and Garmin are very comparable in features. The main differences is that the Garmin has the GPS built in so it has a large form factor and has to be charged frequently, while the Polar’s GPS is an external pod so the main device can be much smaller and you don’t have to charge it. Probably the biggest difference between the two is that the Polar has great software while the Garmin software is terrible. If you go Garmin figure on getting some kind of third party software.